LIBRARY OF CONGRESS, 
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UNITED STATES OF AMERICA. 




-FRONTispiECE.-Fig.l.-Photo^p^of total J ^^^^^feSS 
rbf Professor Pritchett, of Washington University, -St. -Louos, Mo., at -jNennau, 
(Gal. ,on January 4, 1889. 



ASTRONOMY, 



NEW AND OLD. 



/ 



REV. MARTIN S. BRENNAN, A.M., 

Rector of the Church of St. Thomas of Aquin, St. Louis, Mo.; Author of 

" Electricity and its Discoverers, -: and "What Catholics 

Have Done for Science.'" 




NEW YORK: 

THE CATHOLIC PUBLICATION SOCIETY CO., 

9 Barclay Street. 

London: BURNS & GATES. 

1889. 



fir- 






Copyright, 1889, 
By THE CATHOLIC PUBLICATION SOCIETY CO. 



All Rights Reserved. 



CONTENTS. 



Chap. I. History, 7 

II. The Division of Time and the 

Calendar, .... 25 

III. The Spectroscope, ... 40 

IY. The Sim, 46 

V. The Moon, 66 

YI. The Telescope, .... 82 

YII. The Planets, 90 

YIII. Are the Planets Habitable f . 121 

IX. Comets, 129 

X. Shooting-Stars, .... 141 

XI. The Zodiacal Light, . . . 151 
XII. The Starry Heavens : 

1. The Constellations, . . . 156 

2. The Stars, .... 173 

3. Star Clusters and Xebulae, . 193 
XIII. Celestial Photography, . . 203 
XIY. Celestial Laws, .... 207 

XY. Celestial Measurements, . . 218 

XYI. Mechanism of the World, , . 231 

Index, 245 

3 



PREFACE. 



The main object of this book is to give an 
epitome of the vast science of Astronomy in the 
simplest and most concise manner possible. 

The utmost fairness is aimed at throughout, 
and particularly in the treatment of the history 
and different hypotheses of the science. 

There have been for some time past two 
sciences of Astronomy recognized, the " Old " 
and the "New." The majestic Old Astronomy 
traces its descent back through the great Hip- 
parchus to the earliest dawn of tradition. The 
Old Astronomy has been called the most perfect 
of the sciences. It reached its lofty standard of 
perfection, step by step, by subjecting every new 
advance to the rigid requirements of mathe- 
matical accuracy. 

The New Astronomy sprung up beside the 
Old about thirty years ago, and is, really, the 
science of Celestial Physics. The new science 
is more brilliant and fascinating than the old ; 
it is much more winsome, because easier of 
approach. 

However, the new and the old science are daily 



Preface. 



coming closer together. The spectroscope and 
celestial photography lend themselves directly to 
dynamical inquiries, and thus help to form the 
future science of sidereal mechanics, which will 
be the product of the fusing of the "New" and 
the "Old" Astronomy. 

To give an intelligent summary of the great 
science, it is necessary to blend together, in some 
measure, the Old and New Astronomies. Mathe- 
matics, however, is avoided as much as possible, 
and only introduced in its simplest form, and 
when absolutely necessary. 

The major part of the book is devoted to the 
fresh young science of Celestial Physics. 

Every topic of importance in Astronomy is 
treated. A short history of the science is given, 
from its dawn to the present. Considerable space 
is given to that important portion of practical 
Astronomy, the division of time. The principal 
Qses of the two great instruments of the astro- 
nomer, the telescope and spectroscope, are shown. 
The constitutions of the Sun and Planets have 
received careful consideration. The interesting 
subjects of Comets, Shooting-stars, and the Con- 
stellations are not forgotten. Particular attention 
has been devoted to the Zodiacal Light, Celestial 
Photography, the Habitability of the Planets, 
and the great Hypothesis of Laplace. 



THE AUTHOR. 



CHAPTER I. 
HISTORY. 

Asteonomy (dffrpov, a star, and roj.w$, a law) 
teaches all that is known of the heavenly bodies. It 
is the oldest of the sciences, and reaches back to the 
earliest twilight of tradition. 

The Hindoos, Chinese, and Egyptians lay claim to 
a very high antiquity in this study; but it has been 
demonstrated that these claims are utterly unreliable. 
Their cultivation of celestial objects was more senti- 
mental than scientific. The slender data that have 
reached us through them are of the vaguest charac- 
ter, and their crude speculations are entirely devoid 
of system. 

The Chaldeans.— The weight of evidence favors Chal- 
dea as the home of the first students of Astronomy. 
The risings and settings of the heavenly bodies were 
observed by them at a very remote period. They early 
took notice of eclipses, and have left us a catalogue of 
these phenomena. Ptolemy gives the dates of six of 
these events taken from this catalogue, the most an- 
cient going back to 721 B.C. These are the earliest 
reliable observations in existence. 

By noticing the recurrence of eclipses the Chaldeans 
were enabled to discover the lunar period, called by 
them the a saros." The saros is a cycle of 6,585^ days, 
or 223 lunations, after the lapse of which eclipses of 
the same magnitude occur. According to Letronne, 
to the Chaldeans belongs the honor of having invent- 
ed the Zodiac, at the beginning of the sixth century 

7 



Astronomy: New and Old. 



before our era. They determined the equinoctial and 
solstitial points, and are the authors of the duodeci- 
mal division of the day. The clepsydra as a time-piece, 
the gnomon for fixing the solstices, and a hemispherical 
dial for marking the positions of the sun, were in use 
among them. The Chaldeans also determined the 
length of the tropical year to within less than half 
a minute of its true value. 

The Hindoos. — Extravagant claims of a wonderful 
antiquity have been made respecting the astronomical 
attainments of this people. Among their records is a 
statement of a conjunction of the sun, moon, and planets 
observed in 3102 B.C., and their planetary tables are 
alleged to be five or six thousand years old. These 
' tables, however, are not grounded upon any true obser- 
vations, because the conjunctions which they suppose 
could not possibly have taken place. 

Scientific men have proven that this conjunction of all 
the planets was learned, not from any actual record of 
it, but by calculating back the position of the planets. 
The elements of their tables were taken from the Greeks 
and Arabs, as the tables themselves are a mean between 
those of Ptolemy and Albategnius, the Arabian. 

The Hindoos had a very fertile imagination, and a 
fondness for round numbers. For instance, a record of 
theirs, called the " Surya Siddhanta," which they dated 
over a million of years back, Bentley brought down by 
his computations to about the tenth century of our era. 
Schaubach asserts that the astronomical knowledge of 
the Hindoos has been drawn from the Arabs, and is 
consequently modern. Laplace says: "The origin of 
Astronomy in Persia and India is lost, as among all 
other nations, in the darkness of their ancient history. 
The Indian tables suppose a very advanced state of 



History. 9 



Astronomy; but there is every reason to believe that 
they can claim no very high antiquity." 

The Chinese.— The annals of China mention a con- 
junction of the planets Mercury , Mars, Jupiter, and 
Saturn as occurring a century before the Flood. Not 
only is this a legend, but there is the very highest au- 
thority for saying that no truthful records of the Chi- 
nese can claim an antiquity greater than that of the 
foundation of Eome. Among the eclipses reported by 
the Chinese is that of a very ancient one, occurring 
October 13, 2127 B.C., or about 217 years after the 
Flood. All their reported eclipses have been recalcu- 
lated, and but a single one before Ptolemy's time 
could be verified. 

It seems that the Chinese divided the year into 365^ 
days and had an early knowledge of the Luni-solar, or 
Metonic, Cycle of nineteen years. 

Montucla allows that the Chinese discovered the 
equation of the moon's evection and the proper motion 
of the fixed stars (precession) in the third century of our 
era. But the Greeks made these same discoveries five 
hundred years earlier. 

The Chinese have a legend about the tragic death of i 
two of their astronomers, Hi and Ho. They were astro- 
nomers to the emperor, and it was their duty to give 
timely warning of the approach of eclipses, that the pro- 
per religious rites which such occasions demanded might 
be duly performed. Hi and Ho gave themselves up to 
the too frequent use of wine, and neglect of duty fol- j 
lowed. They failed to proclaim the coming of an eclipse, 
and accordingly, the devotional ceremonies being omit- 
ted, China was exposed to the anger of the gods. JSTot 
only is this a fiction, but during the ten succeeding cen- 
turies not a single astronomical fact is found in Chinese 



10 Astronomy: New and Old. 

records. The truth of the matter is, that the Astronomy 
of the Chinese has never advanced beyond a very rude 
and imperfect condition. 

The Egyptians. — Piazzi Smyth claims that the astro- 
nomic knowledge of the Egyptians reaches a high 
antiquity. He points to the pyramids as proof of this. 
He says that the great pyramid was built at the time 
when the star a Draconis would be visible through 
its inclined passage. This would bring its building 
to 2170 B.C. The weight of scientific authority laughs 
at this view of Mr. Smyth. The inscriptions on these 
tombs of Egypt's ancient kings show that they were 
intended solely for neerologic purposes. 

During Napoleon's expedition into Egypt a circum- 
stance occurred that gave that country for a short time 
the reputation of extraordinary astronomic antiquity. 
There were found in an ancient temple at Denderah two 
curiously painted bunds containing the signs of the zo- 
diac. They were called the Zodiacs of Denderah. One 
of these zodiacs was painted in the portico, and the other 
on the ceiling of an upper chamber of the temple. Two 
other zodiacs were found in Esneh ; one painted in a 
small-sized temple, the other in a larger one. These 
were known as the Zodiacs of Esneh. 

From a superficial examination of these zodiacs it 
was concluded that they were executed by some ancient 
Egyptian, and as the zodiacs were supposed to show 
the appearance of the heavens at the time of their exe- 
cution, it signified a fabulous antiquity. Upon a closer 
and more scientific scrutiny, however, it was disclosed 
that the zodiacs were of modern date and astrologic 
character, and belonged to the period when the astrology 
of Rome was introduced into Egypt. 

Egypt was noted for its knowledge of many of the 



History. 11 



sciences before their cultivation in Greece, and many 
Greek philosophers, led by the example of Pythagoras, 
travelled through it to gather wisdom. But Astronomy 
was not one of its flourishing sciences. It is said that 
Thales taught the Egyptians the method of calculating 
the height of the pyramids from the measurement of 
their shadows, and that they informed Herodotus that 
the sun, on at least two occasions, was seen to rise in 
the west. From this it is natural to conclude that 
their astronomic views were not only very limited, but 
at times absurd. 

The Grecians. — With Greece began the real progress 
of Astronomy. Thales (636 B.C.), of Miletus, Ionia, was 
the founder of Greek Astronomy. He is said to have 
computed the sun's orbit, and fixed the year's length at 
365 days. He taught that the earth is a globe, and 
divided it into five zones. By his knowledge of the 
saros of the Chaldeans, he could in a vague way fore- 
cast eclipses. He held that the stars are composed of 
fire, and that the earth is the centre of the universe. 

Anaximander (610 B.C.), of Miletus, Ionia, a pupil of 
Thales, taught, it is said, that the moon shines by 
reflected light, and that the earth turns on its axis. 
He made calculations on the size and distances of the 
heavenly bodies. He computed, for instance, that the 
sun is twenty-eight times larger than the earth. Ac- 
cording to Pliny, Anaximander was the first to teach 
the obliquity of the sun's path. 

Anaxagoras (500 B.C.), of Olazomenge, Ionia, gave a 
correct explanation of lunar and solar eclipses, and con- 
jectured that the moon had hills and valleys similar 
to those of the earth. 

Pythagoras (580 B.C.), of Samos, taught that the 
distances between the planets corresponded to the inter- 



12 Astronomy: New and Old. 



vals of the scale in music, and is the author of the 
"music of the spheres." He was the first to point out 
that Venus is both morning and evening star, and first 
proposed the heliocentric theory, or that the sun is the 
, centre of the planetary system, the earth revolving 
around it. With him, however, it was the merest hy- 
pothesis, and he advanced no proofs to sustain it. He 
was pre-eminently a dreamer, and this was regarded as 
one of his dreams. 

Meton (432 B.C.), of Athens, was the inventor of the 
Luni-solar or Metonic cycle of nineteen years, or two 
hundred and thirty-five lunations. After the lapse of 
nineteen years, the new moons very nearly fall on the 
same days of the year, and eclipses recur in the same 
order relatively to magnitude. 

Calippus (370 B.C.), of Cyzicus, was enabled, by the 
observation of a lunar eclipse, to detect an error of about 
a quarter of a day in the cycle of Meton. He proposed 
to eliminate this error by quadrupling the Metonic cycle 
and subtracting one day. His cycle of seventy-six years 
minus a day is called the Calippic cycle, and was 
adopted in 330 B.C. 

Eudoxus (3G6 B.C.), of Cnidus, was the inventor of 
the theory of Crystalline spheres, or that the heaveuly 
bodies are set in concentric transparent spheres, revolv- 
ing at different distances from the earth. He made the 
earth the immovable centre of the celestial motions, and 
fixed the length of the year at 365J days. 

Alexandrian School-Alexandria, founded by Alex- 
ander the Great (332 B.C.), near the Canopic mouth of 
the Kile, the Greek capital of Egypt, was for about four 
centuries the centre of science in the ancient world. 
Here the culture of the physical sciences reached a 
greater height than anywhere else in antiquity. Its 



History. 13 



astronomers devoted themselves strictly to the work of 
observation. Indeed, all the Greek observations of any 
value begin with the Alexandrian school. 

Aristyllus and Timocharis, tbe earliest Alexandrian 
astronomers, observed the places of the planets and 
fixed stars, and determined the times of the solstices 
(295-269 B.C.) 

Eratosthenes (276 B.C.) attempted the measurement 
of the earth's magnitude by the very method used at 
present, by measuring the length of a degree of latitude, 
and computed with fair accuracy the obliquity of the 
sun's path. 

Aristarchus (281-264 B.C.), of Sainos, endeavored to 
estimate tbe relative distances of the sun and moon 
through means of the angle which they subtend at the 
observer's eye when the moon's face is a perfect semi- 
circle. The want of proper instruments and the impos- 
sibility of knowing when the moon's face is precisely 
half-enlightened rendered his method valueless. He 
estimated the sun's distance at eighteen times the 
moon's, when in reality it is four hundred times. Hum- 
boldt regards him as the pioneer of the Oopernican the- 
ory, as he was said to have regarded the sun as the 
centre of the planetary motions. But if he ever held 
this view, it was in the same sense as Pythagoras, 
simply as a hypothesis. 

Hipparchus, of Nicsea, Bithynia, who flourished in the 
middle of the second century B.C., was the founder of 
scientific Astronomy. He was the greatest observer of 
antiquity, if indeed not the greatest that ever lived. 

According to the ancient system of the world, all the 
heavenly bodies move in a circle and at a uniform rate. 
The planets, however, appeared occasionally to deviate 
from this doctrine, They were perceived sometimes to 



14 * I s tj? onom v : Xe w a xd Old. 

be stationary, sometimes to move on directly, and again 
to even retrograde. To reconcile the actual with the 
theoretic movements, it was necessary to introduce the 
epicycle. Accordingly, the planets were represented as 
moving in circles around fictitious centres, and these 
centres again around the earth. While a planet was 
moving in a small circle, the centre of that small circle 
was describing a larger circle about the earth. The 
laigcr circle was called the deferent, and the smaller, 
which it carried, was called the epicycle. Tims the 
motions of the planets about the earth were thought to 
be similar to what the motion of the moon about the 
sun really is. 

It was also found necessary to imagine eccentrics, 
to explain certain irregularities noticed in the mo- 
tions of the sun and moon. Besides moving circularly 
and uniformly, the heavenly bodies were supposed 
to move around the earth as a fixed centre. The 
sun, however, was perceived to move more rapidly 
in some parts of its orbit than in other parts. This 
could not be if its motion were uniform and the earth 
the centre of this motion. In order to account for this 
want of regularity in the actually witnessed movements 
of the sun, the ancients took the earth away from the 
centre of the sun's supposed orbit. This hypothetical 
orbit of the sun was called the u Eccentric," because its 
centre did not coincide with the earth's centre. It was 
also found necessary to give the moon an eccentric. 

Ilipparchus was the real discoverer and establisher of 
the Theory of Epicycles and Eccentrics, on the princi]>le 
that "he only discovers who proves"; for he demon- 
strated their value and necessity in accounting for the 
motions of the heavenly bodies. 

On this doctrine of epicycles and eccentrics Hippar- 



Histor r. 15 



chus constructed solar and lunar tables, by means of 
which the places of the sun or moon, with respect to the 
fixed stars, could be correctly found at any time. 
These tables enabled astronomers to calculate solar 
and lunar eclipses. This was a severe test of their 
accuracy, for very minute changes in the apparent place 
of the sun or moon would entirely alter the features of 
the eclipse. 

These tables have proved their soundness by credit- 
ably bearing this test for centuries. Hipparchus was the 
discoverer of the precession of the equinoxes, or slow 
motion of the stars away from the equinox, occasioned 
by a slight swaying movement of the pole. He was led 
to this discovery by comparing his own observations of 
the fixed stars with those of Aristyllus and Timocharis. 
Hipparchus made a very reliable catalogue of 1,081 
stars. 

Ancient writers have spoken of the great Bithynian 
with highest admiration. The moderns have praised 
him with equal fervor, and even the severe Delambre 
says : "In Hipparchus we find one of the most extraor- 
dinary men of antiquity ; the very greatest, in the sci- 
ences which require a combination of observation with 
geometry." The works of Hipparchus are lost, and he 
is known only through his successor, Ptolemy. 

Ptolemy, who flourished in Alexandria about 139 
A.D., is the only authority in existence on ancient 
Astronomy, and on this account has given his name to 
the old system of the world. His great work, the 
Almagest, was the standard text-book on Astronomy for 
fourteen centuries. He invented a Planetary Theory, to 
account for the motions of the planets, and also discov- 
ered the Moon's Evection, or swaying. Ptolemy was the 
last of the great observers of Alexandria. 



J 6 Astronomy: New axd Old. 

Arabians.— The culture of Astronomy passed from 
Alexandria into Arabia. Arabian Astronomy began 
about 702 A.D., and flourished during four centuries. 

The Arabians, though careful and assiduous observ- 
ers, added but little to the growth of the science. They 
followed too reverently the methods of their Greek 
masters. Their observations are valuable, because they 
were made with more skill and better instruments than 
those of the Greeks. It may be, too, that the deep azure 
ol* Arabia's sky and the dryness of its atmosphere en- 
hanced their precision. They obtained a truer value for 
the obliquity of the sun's path and the inclination of 
the moon's orbit. 

Albategnius (880 a.d.) was the greatest of the Ara- 
bian observers. 11(5 published tables of the motions of 
the sun, moon, and planets, which were improvements 
on those of Hipparchus and Ptolemy. 

Ebn Junis (1000 a.d.), another distinguished Arabian 
astronomer, also published tables of the sun, moon, and 
planets, which were called the " Hakemite Tables," in 
honor of the reigning caliph, Hakem. Ebn-Junis 
recognized the perturbations of the orbits of Jupiter 
and Saturn. 

A number of other Arabians gave their names to 
tables of the motions of the heavenly bodies. 

The Arabian astronomers made praiseworthy efforts 
to rectify the older tables, by a comparison with the 
heavens. 

Some Arabic terms are still retained in text-books of 
the science, as zenith anil nadir, azimuth circles and 
almanac ; also such star names as Aldebaran, Eigel, and 
Fomalhaut. 

Copernican System.— The onward march of Astronomy 
reached Europe in 1230 A.d., when the Almagest was 



History. 17 

translated under Frederick II., of Germany. And in 
1252 a.d. astronomical tables were formed at the instance 
of Alfonso X., of Castile. Nicholas of Cusa, almost a 
hundred years before Copernicus, ascribed to the earth 
both rotation on its axis and translation in space. 
Two distinguished European astronomers, followers of 
Ptolemy, George Purbach and Begioinontanus, were the 
immediate predecessors of Copernicus. 

We now reach Copernicus (1173-1543), who gave his 
immortal name to the modern system of the world. He 
was a native of Thorn, in Prussia, and having distin- 
guished himself in mathematics while a pupil of the 
University of Cracow, went, at the age of twenty-five, 
to Borne to study Astronomy under the renowned Begio- 
montanus. While in Borne he received holy orders, and 
going back to his native land, was given the poor 
canonry of Frauenburg, near the mouth of the Vistula. 
He spent nearly forty years in assiduously observing 
the heavens and meditating upon its mechanism. 

He was a thorough astronomer, and studied nature 
deeply. He tells us himself that he was first led to 
suspect the Ptolemaic system by its striking want of 
simplicity and symmetry. The eccentrics and epicycles 
had so grown in number as to become enormously 
cumbersome. The more he studied nature, the more he 
became convinced of the simplicity and symmetry of her 
conduct. Driven from the Ptolemaic system, which 
made the earth the centre of motion, he tried the helio- 
centric, which places the sun at the centre of the celes- 
tial movements. Having chosen his hypothesis, he fer- 
vently devoted his great energies to the task of its 
establishment. 

He soon satisfactorily accounted for the diurnal 
revolutions of the sun, moon, and stars, the slow 



18 Astronomy: New and Old. 

progress of the planets through the signs of the Zodiac, 
and the numerous irregularities to which the planetary 
motions are subject. He studied long and patiently 
apparent motion in all its aspects. When rapidly mov- 
ing by a headland on a sailing vessel we lose the con- 
sciousness of our own motion, and see the shore recede. 
Trees and other objects appear to glide by us when 
riding swiftly past them. So the actual revolution of 
the earth on its axis in twenty-four hours from west to 
cast produces the apparent diurnal movements of the 
sun, moon, and stars from east to west. The leading 
tenets of the Copernican doctrine are: 1. The earth 
revolves on its axis, daily producing an apparent revolu- 
tion of the heavens. 2. The sun is the centre around 
which the earth and planets revolve from west to east. 
He clearly explained how the apparent annual revolu- 
tion of the sun among the stars could be produced by 
the annual revolution of the earth around the sun. 

Copernicus, however, continued to believe with the 
ancients in the circular motion of the heavenly bodies, 
and so was obliged to retain a portion of their epicycles 
and eccentrics. He had but very primitive instruments 
to aid him in his grand revolution, and so did not add 
much to observational astronomy. His greatest glory is 
to have clearly pointed out the right path, though he 
himself did not travel far along it. His great work, On 
the Revolutions of the Heavenly Bodies, was published 
at the earnest entreaty of his friend, Cardinal Schom- 
berg, and the first printed copy was put in his hands a 
few days before his death. 

Tycho Brahe (1546-1601), of Knudsthorp, Denmark, 
was one of the most indefatigable observers that ever 
lived. His observations were both abundant and accu- 
rate, and it was mainly through the means of his accu- 



History. 19 

rate work that Kepler was enabled to discover Ms great 
laws. 

Tycko did not give his assent to the Copernican 
doctrine, but originated a system of his own. He main- 
tained that the earth is the centre of the universe, 
around which the whole host of heaven make a 
diurnal revolution; that the moon revolves about the 
earth as the centre of her motions ; that the planetary 
bodies revolve around the sun as the centre of their 
motions, and that the sun, carrying with him the 
planets, travels annually around the earth. 

Among his many discoveries were those of the varia- 
tion and annual equation of the moon. He made the 
first table of refractions, and compiled a catalogue of 777 
fixed stars, a more perfect catalogue than any previous 
one. He also left some valuable observations of comets. 
To aid him in his astronomic labors he had a magnifi- 
cent observatory and many beautiful and rare instru- 
ments, the workmanship of his own ingenuity. He has 
been called the founder of modern astronomical calcula- 
tions. 

Kepler (1571-1630), of Magstadt, Wiirtemberg, dis- 
covered the three remarkable laws bearing his name 
that govern the motions of the heavenly bodies. He 
assisted Tycho Brahe in his observations, and was an 
enthusiastic advocate of the Copernican theory. It was 
his efforts to reconcile the various positions of the planet 
Mars, as observed by Tycho Brahe, with the Copernican 
doctrine of circular motion around the sun that led 
Kepler to the discovery of his laws. The first law is, 
that the planets move in ellipses, with the sun as one 
of the foci ; the second, that a line joining the planet 
and sun sweeps over equal areas in equal times ; and 
the third, that the square of the times of revolution 



20 Astronomy: New and Old. 

of the planets is x^roportioned to tlie cube of their mean 
distances from the sun. 

Kepler was a man of the most indomitable persever- 
ance. The discovery of his third law took twenty-two 
years of the most vigorous application. He wished to 
establish some relationship between the time of revolu- 
tion and the mean distance of the planet from the sun. 
He worked on the problem for twenty -two years before 
solving it, and although he failed again and again in its 
solution, still he persevered. The glory of his discov- 
eries is much enhanced when we consider the difficulties 
lie had to contend with. Modern Astronomy was in its 
infancy, the instrumental aids were scanty, and the 
reliable facts meagre; add to this that his life was one 
of peculiar sadness and singular hardships, and an ever- 
lasting struggle with poverty. Kepler freed the Coper- 
niean system of eccentrics and epicycles. He established 
the law of the diminution of light in proportion to the 
inverse square of its distance. He calculated the times 
of the transits of Mercury and Venus across the sun's 
disc, [reaffirmed that bodies attract in proportion to 
their mass. Both Kepler and Copernicus had a notion 
of the law of gravitation. John Kepler was one of the 
greatest astronomers of all ages. 

Galileo (1564-1642), of Pisa, the discoverer of " the 
three laws of motion," and called the father of experi- 
mental science, was the first to apply the telescope to 
the study of the heavens. His grand discoveries 
rapidly dissipated all doubt regarding the truth of the 
Copernican system. 

The telescope enabled him to see, in quick succession, 
the inequalities of the moon's surface, the spots on the 
sun, the satellites of Jupiter, the appendages of Saturn, 
and the phases of Yenus. The discovery of the Jovian 



Histor r. 21 

system to be a perfect miniature of the Solar system, 
and that of the phases of Venus showing it to be an 
interior planet, may be said to have finally established 
the Copernican doctrine. 

Newton (1642-1727), Woolsthorpe, Lincolnshire, dis- 
covered the great law of universal gravitation : that 
the force of gravity is proportioned to the quantity of 
matter and inversely as the square of the distance. 
Jean Picard's accurate measure of the earth's radius, in 
1670, furnished Newton the means of establishing this 
great fundamental truth. 

Mathematics and mechanics now commenced a won- 
derful growth, and Astronomy, with their aid, made 
rapid strides in progress. 

Schemer, Huyghens, Leibnitz, Dominicus Cassini, 
Halley, Bradley, Lalande, Lagrange, Delambre, Laplace, 
and Herschel greatly improved the science of Astronomy 
by their valuable researches. 

On the first evening of the nineteenth century Piazzi 
discovered the first of the Asteroids, or minor planets. 
This opened a new field of discovery, which was entered 
with avidity. Planetoid after planetoid was found in 
rapid succession, until 281 of these small bodies are 
now known to Astronomy, revolving in the great void 
between Mars and Jupiter. 

Leverrier (1811-1877), of Saint-L6, performed the 
greatest astronomic feat of the century. He calculated 
the mass and orbit of the planet Neptune from the 
perturbations it occasions in the path of the planet 
Uranus. So precise were his figures that when the 
telescope was turned, on the 23d of September, 1847, to 
the place in the heavens indicated by Leverrier, the 
planet was found within one degree of the computed 
point. This brilliant achievement demonstrated the 



22 Astronomy: New and Old. 



high perfection to which the science had readied. 
Leyerrier's theory and tallies of each one of the major 
planets is one of the greatest works ever accomplished 
by an astronomer, and occupied thirty-four years of 
incessant labor. 

In recent years America has given to Astronomy 
some of its brightest names. The recent progress of the 
science has been materially assisted by the contributions 
of Professor Holden ; S. W. Burnham, the great double- 
stars resolverj Asaph Hall's discovery of the satellites 
of Mais and his star parallaxes ; Kirkwood's lucid theory 
of comets and meteors and law of the distribution of 
the asteroids; the elder Bond's discovery of Saturn's 
dusky ring ; the labors of the younger Bond and Peirce 
on the nature of the structure of Saturn's rings ; and the 
valuable researches on the Corona, the distance of the 
sun, the lunar theory, and formation of the planets by 
the Nestor of American astronomers, Professor Simon 
Newcomb. 

The momentous discovery of Spectrum Analysis has 
created a "tfew Astronomy," grown up beside the Old. 
This New Astronomy studies the sun, moon, and stars 
for what they are in themselves and in their relation to 
us. It studies their chemistry or physical constitution, 
and hence it is also called Solar Physics or Celestial 
Physics. Spectrum analysis is a mode of distinguishing 
the various species of matter by the kind of light pro- 
ceeding from each. The most obvious distinction be- 
tween one kind of light and another resides in the color. 
In the refracting prism or spectroscope there is a scale 
of color. Substances have their characteristic place in 
this scale. The spectroscope is a wonderful instrument 
in the hands of the astronomer. It recognizes the 
presence and condition of matter in the face of almost 



History. 23 



infinite minuteness and almost infinite distance. It will 
detect the one-eighteen-millionth of a grain of sodium in 
the flame of a spirit-lamp, and reliably indicate the 
material composition of the sun or even of the faintest 
star. The spectroscope shows that many of our familiar 
elements exist in the sun in the vaporous state ; that the 
stars are incandescent globes en wreathed with glowing 
vapor containing some of our known elementary sub- 
stances ; that some of the nebulae are resolvable into 
stars, and some are glowing gas mostly composed of 
hydrogen and nitrogen. The spectroscope also gives an 
approximate measure of the motion of the stars. 

Among the great workers in the field of the New 
Astronomy distinguished places are due to Fraunhofer, 
Kirchhoff, Huggins, Secchi, Respighi, Lockyer, Zollner, 
Janssen, and our own illustrious three — Draper, Young, 
and Langley. 

Celestial Photography is a great aid to the astrono- 
mer of to-day, and has made wonderful progress within 
a very few years. The chemical plate is sensitive to 
rays which are incapable of affecting vision, and by long 
exposure can accumulate impressions almost indefinite- 
ly. Faint objects may thus be photographed which 
the telescope could never reveal. Photography also 
registers planetary, solar, and stellar phenomena in- 
dependent of the source of error of ordinary obser- 
vation. 

Stellar Photometry, which treats of the measurement 
of the intensity of star-light, has reached such excel- 
lence that it is now cultivated as a separate branch 
of Astronomy. Professor Pickering has done much 
valuable work in this field, and has already constructed 
a photometric catalogue giving a careful measurement 
of the brightness of 4,260 stars. 



~ 4 Astronomy: New and Old. 



Electricity has found its way into the observatory, 
and has been of great advantage in many ways to the 
observe]-. In Astronomy it is of the highest impor- 
tance to be aide to measure exactly extremely small 
intervals of time. The revolving cylinder and electric 
marking apparatus divide a second into a thousand 
equal parts, 



CHAPTER II. 
THE DIVISION OF TIME AND THE CALENDAR 

THE DIVISION OF TIME. 

One of the earliest purposes of Astronomy was to 
afford a means of measuring time. The astronomical 
divisions of time are the day, the month, and the year. 
Indeed, these three divisions have been in all ages the 
fundamental units of time, the first being measured by 
the revolution of the earth on its axis, the second by 
that of the moon around the earth, and the third by 
that of the earth around the sun. 

The day is the most striking and best marked division 
of time. Man in a very primitive condition must have 
been able to form a conception of the day as a measure 
of time. The recurrence of light and darkness, warmth 
and cold, noise and silence, the rising and setting of the 
sun at almost equal intervals, must have arrested the 
attention of the rudest people. And the intervals of 
repetition are so short as to be capable of being grasped 
by the weakest memory. The alternation of day and 
night, occurring with such uniformity, furnished in all 
ages the most definite unit of time. 

There are two kinds of days, a sidereal and a solar 
day. A sidereal day is the length of time it takes the 
earth to turn on its axis relatively to a fixed star. A 
solar day is the time of revolution relatively to the sun, 
and is the interval of time which elapses between two 
successive passages of the sun over the meridian. The 
solar is nearly four minutes longer than the sidereal 



26 Astronomy: Xew and Old. 

day. The difference is occasioned by the earth's motion 
of translation around the sun. Whilst the earth is 
turning on its axis, it is at the same time pushing on in 
its orbit at the rate of nineteen miles a second. If the 
earth had remained fixed in space, the sun and stars 
would reappear at the same time in the meridian, the 
sidereal would have been of the same length as the 
solar day. But the earth is not fixed, it has travelled 
onward to another point. The star, because it is 
situated at an almost infinite distance, is again found, 
after a complete rotation, in the meridian; but the sun, 
its distance being appreciable, is thrown a little out of 
position, and the earth must turn through a space of 
one degree of arc, or four minutes of time, to bring the 
sun to the meridian. 

Another explanation is this : The real westward 
motion of translation of the earth in its orbit gives the 
sun an apparent eastward motion among the stars. In 
one year, or 3G5^ days, the sun apparently travels east- 
ward around the whole heavens, or over 360 degrees 
of an arc. It thus moves eastwardly nearly one degree 
a day. While, therefore, the earth is turning on its 
axis, the sun is moving in the same direction, so that 
when we have come round under the same celestial 
meridian from which we started, we do not find the sun 
there, but he has moved eastward nearly a degree, and 
the earth must perform so much more than one complete 
revolution before we come under the sun again. Now, 
since we move, in the diurnal revolution, fifteen degrees 
in sixty minutes, we must pass over one degree in four 
minutes. It takes, therefore, four minutes for us to 
catch up with the sun after we have made one whcle 
revolution. Hence the solar day is about four minutes 
longer than the sidereal day. 



Division of Time axd the Calendar. 27 

The earth's path around the sun is not a circle ; it 
is an ellipse, and so the real motion of the earth in its 
orbit, or the apparent motion of the sun in its path, 
is not uniform. Again, the variable inclination of the 
sun's path to the equator is another cause of irregularity 
in the sun's apparent motion. Consequently, owing 
to these causes, the solar days do not always differ from 
the sidereal days by these four minutes. The daily 
time by the sun is called apparent time. Mean time 
is the average length of all the solar days throughout 
the year. The mean or average day is the civil day, 
and consists of twenty-four hours, beginning at mid- 
night, when the sun is on the lower meridian. 

The astronomical day is the apparent solar day 
counted through the whole twenty-four hours, and 
begins at noon. Astronomers make most of their 
observations on the meridian, and are mostly looking 
towards the south ; and left with them means east, and 
right west. Geographers, on the other hand, regard 
the right as east and the left as west, because formerly 
they were more familiar with the northern than the 
southern hemisphere, and were thought to look towards 
the north. 

Civil or mean solar time is the time kept by clocks. 
This mean solar time, as already mentioned, differs 
continually from apparent time, or the time marked 
by the sun. The sun is sometimes fast and sometimes 
slow. The difference between mean time and apparent 
time is called the equation of time. Almanacs contain 
the equation of time for every day in the year. As no 
clock, however perfectly constructed, can be relied upon 
to run to true mean time, or to any exact definite rate, 
therefore clocks must be frequently rectified by the sun. 
We can observe the apparent time by the transit 



. J N TR NOM ) ' : Xe W A ND OLD. 



instrument, and then, by the application of the equation 
of time, we determine the true mean time. When the 
sun is fast, it comes to the meridian before twelve 
o'clock, true mean time; and when slow, it comes to the 
meridian later than twelve o'clock. The amount, fast 
or slow, for each day is found in the table of the 
equation of time. When the sun is fast we must sub- 
tract the equation of time from apparent time to obtain 
mean time; and conversely when the sun is slow. 

it happens 2 however, four times in the year that the 
mean and apparent times are equal to each other; 
namely, April the 15th, June the 16th, September the 
1st, and December the 24th. Their greatest difference 
is on Uiu :>d of November, when the apparent is 
K> minutes and 17 seconds greater than the mean 
time. 

.Menu time is so measured that the hours and days 
shall always be of the same length, and shall, on the 
average, be as much ahead of the sun as behind it. 
Corrections are constantly made at the chief obser- 
vatories, and time-signals wired at fixed hours to all 
points throughout the country. 

Next to the day the most natural division of time is 
the year. The notion of a year arose, in the same 
manner as that of a day, from the recurrence of certain 
Tacts after certain intervals. Indeed, in all parts of the 
world the yearly cycle of changes has been singled out 
from all others, and designated by a peculiar name. 
The Latin term for year signifies a ring, the Greek- 
implies something which returns into itself, and our 
own word year is derived from the Swedish of ring. 
A year is the period of the revolution of the earth 
around the sun. The sidereal year is the period re- 
quired by tha sun to move from a given star back to the 



Divisiox of Time axd the Calexdar. 29 

same star. It consists of 365 days, 6 hours, 9 minutes, 
9.0 seconds. The tropical year is the time which elapses 
from the sun's appearance on one of the tropics to its 
return to the same, and has an average length of 305 
days, 5 hours, 48 minutes, 49.7 seconds. The tropical 
year being the one commonly applied to the measure of 
time, is also called the civil year. Owing to the preces- 
sion of the equinoxes, or the retrogradation of the 
equator on the ecliptic, the tropical is less than the 
sidereal year by about 20m. 20s. 

The most ancient nations determined the number of 
days in the year by means of the stylus or perpendicular 
rod, which casts its shadow along a level plane bearing 
a meridian line. The shadow was seen to be shortest 
at the summer solstice; and the number of days that 
elapsed until the shadow was again the shortest was 
found to be 365 ; and this period was adopted for the 
civil year. But the real length of the year is very 
nearly 305 days and a quarter. If a year of 305 days 
were used, in four years the year would begin a day too 
soon. At the end of four more years it would begin 
two days too soon ; and in the progress of time the civil 
year would be found not to coincide with the year of 
the seasons, and thus all dates would be thrown into 
confusion. 

The Julian year, which we now use, consists of 365 \ 
days, and takes its name from its adopter, Julius Caesar. 

The Egyptians knowingly permitted their civil year 
to wander, as they wished their religious festivals to go 
through all the seasons of the year. In 1,461 years the 
festivals would make a circuit of the seasons ; for 1,400 
years of 305^ days are equal to 1,401 years of 365 days. 
This period of 1,461 years is called the Sothic Period, 
from Sothis, the Egyptian name of the Dog-star. 



30 Astronomy: New axd Old. 

The Egyptians corrected their time by the rising of 
Shins, or the Dog-star. 

The number of days in a year is too great to be 
easily remembered, and so a middle measure between 
fche day mid year was desired. The phases of the moon 
suggested such a measure. It was noticed that new 
moons succeeded one another after average intervals of 
29] days. Thus the age of the moon furnished a con- 
venient measure of time. The moon travels from one 
point in the heavens back to the very same position in 
29] days. These 29j days constitute a lunation or lunar 
month. The actual length of a lunation is 29 days, 12 
hours, 44 minutes, and 3 seconds. In that time the 
moon returns to occupy the same position with respect 
to the sun and earth. But the moon makes a sidereal 
revolution of its orbit, or moves from a fixed star back 
to the same fixed star, in 27 days, 7 hours, 43 minutes, 
and 11J seconds. This difference of more than two days 
is caused by the motion of the earth in its orbit. 
While the moon is going around her orbit the earth has 
travelled quite a distance in her own orbit, and it takes 
the moon a little over two days to pull up this differ- 
ence. 

The Greeks and other ancient nations made the year 
depend on the course of the moon. They supposed that 
twelve lunations were equal to one revolution of the 
sun. Consequently their year numbered 354 days, 
being 12 months of alternately 29 and 30 days each. 
But this year differed from the year that governed the 
seasons by more than 11 days. This difference between 
the year of the seasons and the civil year would gradu- 
ally widen and be a source of much contusion. Efforts 
were made to rectify the matter by introducing a 
number of days into the civil year, to make it agree 



Division of Time and the Calendar- 31 

with the course of the sun. But the additions, or 
intercalations, as they were called, were often omitted 
and frequently made at pleasure, and so the year was 
sometimes long and sometimes short. 

The difficulty of making any specific number of lunar 
months correspond with the sun's course was so great 
that the lunar month was generally abandoned, except 
by those people whose religious ceremonies depended 
on the time of new moon. Among those who aban- 
doned lunar months the year has been divided into 
twelve months of slightly different lengths. 

The Greeks, who retained the lunar months on 
account of their religious rites, had great difficulty 
with their dates, until the Athenian mathematician, 
Meton, in 432 B.C., discovered the Metonic Cycle of 
nineteen years, which is so correct and convenient that 
it is in use to this day. Meton discovered that 19 
years are almost equal to 235 lunations. 235 lunations 
are greater than 19 true tropical years by 2 hours 
and 4 minutes. And 19 Julian years are greater than 
19 true tropical years by 3 hours and 33 minutes. 
Hence, if the 19 years be divided into 235 months, so 
as to agree with the changes of the moon, at the end 
of that period the same succession may begin again 
with great exactness. 

Calippus, in 330 B.C., by observing an eclipse of the 
moon, discovered the error of the Metonic Cycle, and 
slightly corrected it by leaving out a day at the end 
of four of Meton's cycles. Four Metonic cycles are 
equal to 76 years, and these constitute the Calippic 
period. 

The week, as a division of time, has come to us from 
the Mosaic dispensation. The Jews, however, had no 
special name for the single days, but counted their 



32 Astronomy: New and Old. 

number from the previous Sabbath. Thus Sunday was 
the first and Friday the sixth day of the week. Satur- 
day, Sunday, and Monday derive their names obviously 
from Saturn, the Sun, and the Moon. Tuesday, Wed- 
nesday, Thursday, and Friday borroAV their names from 
the Saxon of Mars, Mercury, Jupiter, and Venus. 



THE CALENDAR. 

The arrangement of the divisions of time with us, 
or our calendar, has come from ancient Rome. In the 
reign of Nuina the Roman civil year consisted of 355 
days. This king, in order to compensate for the short- 
age in the civil reckoning, ordered the insertion, after 
certain intervals, of an intercalary addition to the year. 
This correction was frequently omitted for political 
reasons, and, in the consulate of Julius Caesar, the 
direst confusion had crept into the Roman dates. The 
civil year had grown 90 days too long. Caesar called 
upon Sosigenes, an astronomer of the famous school of 
Alexandria, to assist him in the formation of a calendar. 
By the advice of the latter, the new Roman calendar 
was ordered entirely by the motion of the sun. The 
year 4G B.C. was made to consist of 455 days. This was 
done in order to throw away the 90 days that had 
crept into the civil year through the errors of the old 
system of reckoning. The year 45 B.C. was the first 
regular year under the new calendar, and began on the 
1st of January, it being the day of the new moon imme- 
diately following the winter solstice. The Romans were 
the first to adopt the 1st day of January as the first 
of the year. 

According to the Julian calendar the year was to 



Division of Time and the Calendar. 33 

consist of 365J days, so contrived that the year would 
ordinarily consist of 365 days, and every fourth year 
would be bissextile, or leap-year, consisting of 366 days. 
It was called bissextile because the 6th of March was 
doubled. The Julian rule was, however, an over- 
correction. It made the year consist of 365 days and. 6 
hours, when in reality the length of the true tropical 
year is 365 days, 5 hours, 48 minutes, and 50 seconds. 
The Julian year is too long by 11 minutes and 10 sec- 
onds, and would occasion an error of a week in about a 
thousand years. The error reached ten days in the 
sixteenth century, and caused some confusion in regard 
to the time of celebrating Easter. 

Pope Gregory XIII. rectified the matter by reform- 
ing the calendar. He ordered that the 5th of October, 
1582, should be reckoned the 15th of October. And in 
order to avoid future confusion, he ordained that every 
hundredth year should not be counted a leap-year, 
excepting every four-hundredth, beginning with the 
year 2000. 

The Gregorian rule is as follows: The years are 
numbered from the Birth of Christ. Every year whose 
number is not divisible by 4 without a remainder, 
consists of 365 days; every year which is divisible 
by 4, but not by a hundred, consists of 366 days; 
every year divisible by 100, but not by 400, again 
of 365 days; and every year divisible by 400, again 
of 366 days. 

The actual value of the solar year, reduced to a 
decimal fraction, is 365.24224 ; and of the Gregorian 
365.2425, so that the error of the Gregorian rule on ten 
thousand tropical years is 2.6 days. This is an error of 
less than a day in three thousand years. The rule is 
sufficiently accurate for all civil reckoning, 



34 Astronomy: New and Old. 



In order to form a perfect and convenient calendar, 
the units of measurement .should be invariable and coin- 
mensurable. Nature has forced upon us as units of 
time-measure the solar day and tropical year. The 
most unchangeable thing in nature is the length of the 
mean solar day, which may indeed be said to be ab- 
solutely invariable, as it has not varied the hundredth 
pari of a second within historic times. The tropical 
year, owing to a retrogradation of the ecliptic on the 
planetary orbits, has shortened 4.21 seconds since the 
time of Hipparchus. Nevertheless, it is sufficiently in- 
variable for all purposes, and particularly as the day 
is, in effect, the real standard unit. All the troubles 
of the calendar, then, arose from the incommensurability 
of the tropical year and the solar day. 

The Golden Number.— Meton discovered that 235 luna- 
tions brought the moon back to where she was 19 years 
before. Thus, new moon occurred on new year's day 
in 1861 ; it did not occur on the same day until after 
a lapse of 1!) years, or until 1880; and it will fall on 
this same day every 19 years for about 400 years. This 
period of It) years is hence known as the Metonic or 
Lunar cycle. The Golden Number is the number of this 
cycle corresponding to the current year. The golden 
numbers range from 1 to 1!). The number of each year 
in the Metonic cycle was formerly ordered by the Greeks 
to be engraved in letters of gold on marble pillars, and 
hence the origin of the name. 

Under tha Gregorian rule the golden number is reck- 
oned from the year 1 B.C. In that year the new moon 
came on the 1st of January, and has come on new year's 
day every 10th year since. To find the golden number 
tor any year, add one to the number of the year, divide 
the sum by nineteen, and the remainder will be the 



Drrrsiox of Time and tee Calexdar. 35 

golden number for that year. If there be no remainder, 
then nineteen is the golden number of the year, or the 
Last of the cycle. 

Epact.— The Epact (derived from a Greek word mean- 
ing to bring on or in) is the age of the moon on new 
year's day. On new year's day, - in the year 1 B.C., the 
calendar year and the lunar year began together. On 
the next new year's, the lunar year having run out 
eleven days sooner than the calendar year, the moon was 
eleven days old, the calendar being (nearly) 365 and the 
lunar year 354 days. Thus eleven days was the moon's 
age on new year's day of that year, or that year's epact. 
In the next year there was a difference of twenty-two 
days between new moon and new year's day, and so on. 

To find the epact for any year, divide the year by 
nineteen, the Lunar cycle, multiply the remainder by 
eleven, the excess of the calendar above the lunar 
year, and divide the product by thirty, the number of 
days in a mean calendar month ; the remainder will be 
the epact of that year. Or a shorter method is to 
subtract one from the golden number of the year, mul- 
tiply the remainder by eleven, divide the product by 
thirty, and the remainder will be the epact. Should 
there be no remainder, the age of the moon will be 
twenty-nine and a half, the number of days in the 
average lunar month. 

To find the date of the January new moon, the epact 
must be subtracted from 29 £ days, the length of a luna- 
tion. The dates of the successive new moons, in any 
year, can be found by adding 29 and 30 days alter- 
nately to the date of the January new moon. When 
we thus have the date of the January new moon for any 
year, we can easily find the date of any new or full moon 
throughout that year. 



36 Astronomy: New and Old. 

Easter. — Easter is the Sunday following the first full 
moon after the vernal equinox. This time for the 
Easter celebration was fixed by the Council of Nice, 
held in the year 325 a.d. In that year the vernal 
equinox fell on the 21st of March. In the Gregorian 
calendar the vernal equinox falls on the 21st of March, 
and the " full moon " is the 14th day of the calendar 
moon. 

Easter, then, is always the first Sunday after the 
full moon which happens upon or next after the 21st 
of March. If the full moon fall on the 21st of March, 
and this be a Saturday, the next day will be Easter 
Sunday. If the full moon happen upon a Sunday, 
Easter Day is the Sunday after. If the full moon fall 
on the 20th of March, this moon cannot be considered 
the Paschal moon ; the next moon only, which will be 
on the 18th of April, can be reckoned such. Should 
this 18th of April be a Sunday, Easter would be the 
Sunday following, or the 25th of April. Easter, then, 
can be no later than the 25th of April, nor earlier than 
the 22d of March. Easter determines the times of all 
the movable feasts of the Christian Church. 

To determine the time of Easter for any year, find, 
through means of the golden number, the epact, or the 
moon's age on the new year's day of the year ; subtract 
this from 29J days to find date of the January new 
moon ; add alternately to this date 29 and 30 days for 
each moon, until the full moon immediately succeeding 
March 21 is found; the following Sunday is Easter. 

The Solar Cycle. — It is often desirable to know the 
day of the week on which an event happened, when 
only the day of the month and year are given. After 
the lapse of a certain period it is found that the same day 
of the month recurs to the same day of the week. This 



Division of Time and the Calendar. 37 

period is called the Solar Cycle, and consists of 28 
Julian years. If the year consisted of exactly 52 
weeks, or 364 days, the day of the week and 
the day of the month would always correspond $ and 
if Monday were the first day of the year, it wonld be the 
first day of every year for all time to come. But the 
year consists of 365 days, or one day more than 
52 weeks. Hence, any day of the month is one day 
later in the week than the corresponding day of the 
preceding year. If the year begins on Tuesday, we 
should complete 52 weeks on Monday, leaving one day, 
Tuesday, to complete the year, and the following year 
would begin on Wednesday. So that every day of the 
week goes back, every year, one day from the date 
it occupied on the previous year. Leap-year, however, 
consists of 52 weeks and two days 5 and all the days 
of the week in the succeeding year, and from February 
29 of leap-year, go back two days. If the year 
consisted regularly of 365 days, at the end of seven 
years the days of the month and week would again 
correspond. Leap-year comes in every fourth year, 
however, as a disturbing element, and prevents this 
coincidence of the day of the week and the day of the 
month after seven years 7 intervals. We have here 
intervals of seven years and of four years that must 
be reconciled. This reconciliation is effected by taking 
a period corresponding to their common multiple, 28. 
The coincidence between the days of the week and the 
days of the month regularly recurs after 28 years. The 
Solar cycle is arranged so that it takes as its starting 
point the year 9 B.C., and is thought to have been 
invented about the time of the Council of Mce (325 
a.d.) The leap-years in the Solar cycle are computed 
according to the Gregorian rule. 



38 Astronomy: New and Old. 

Cycle of Indiction. — This is a cycle of 15 years, and 
owes its origin to the fact that the Roman emperors 
issued their edicts for tlie collection of the tribute every 
fifteen years. It is frequently mentioned in ecclesias- 
tical history, and is reckoned from the 1st of January, 
313 A.D. 

The Julian Period. — This is a compound cycle, and 
consists of 7,980 years. It was invented by Joseph 
Scaliger, and is the least common multiple of the Solar, 
Lunar, and Indiction cycles. Its purpose is to have 
some standard epoch to which the chronological reckon- 
ings of the different nations may be referred. A hypo- 
thetical date, 4713 B.C., is fixed as the period of its 
commencement. The Julian period for any year is 
found by adding 4,713 to that year. The Julian period 
for 1888 is 4,713 pins 1,888, or G,G01. The Solar cycle 
for any year is found by dividing its Julian period by 
28 ; the remainder is its Solar cycle. When the remain- 
der is the Solar cycle is 28, because it is the last year 
of the cycle. The Metonic cycle for any year is found 
by dividing its Julian period by 19; the remainder is its 
Metonic or Lunar cycle. The Indiction for any year is 
found by dividing its Julian period by 15 ,• the remain- 
der is its Indiction. 

Dominical Letter. — The early Christians introduced 
the custom of placing the first seven letters of the 
alphabet, A, B, 0, D, E, F, and G, in the order of their 
succession, against the days of the month. The 1st 
of January was represented by A, the 2d by B, the 3d 
by 0, and so on to Gr, when the seven letters began 
over again, and were repeated through the year in the 
same order. The letter that falls against the first 
Sunday in January- in any year is called the Sunday 
or Dominical Letter of the year, and will fall against 



Dtvtsion of Time: and the Calendar. 39 

every Sunday in that year if it be a common year. An r 
exception occurs in leap-years, when February 29 and 
March 1 are marked by the same letter, so that a 
change occurs at the beginning of March. In leap- 
years there will be two dominical letters, that for the 
last ten months of the year being the one next preceding 
the letter for January and February. Any day of 
a past year occurred one day earlier in the week for 
every year that has elapsed, and, in addition, one day 
earlier for every 29th of February that has intervened. 

The dominical letter, being known for any one year, 
can be found for any other by simply remembering that 
an ordinary year is 52 weeks and one day, a leap-year 
52 weeks and two days, so that the dominical letter will 
go back from Gr towards A one letter for a common 
year and two letters for a leap-year. The Solar cycle 
brings back the days of the week to the very same days 
of the month. Hence the days of the week fall upon 
the same days in the same month every 28th year. So 
that every 28th year has the same dominical letter. 

To find the dominical letter for any year of the nine- 
teenth century, take the number of the year and add r 
one quarter of itself, neglecting fractions, and divide the= 
sum by seven ; then subtract the remainder from 8, or, . 
if it is 0, from 1, and the new remainder will be the- 
number of the dominical letter in the alphabet. If it 
be a leap-year, the dominical letter thus obtained' 
belongs to the time after February 29, and for the- 
preceding two months the dominical letter will be the' 
succeeding letter in the alphabet. In computing the 
dominical letter for other centuries, it must be remem- 
bered that, under the Gregorian rule, 1700, 1800, 2100, 
and so on, are not leap-years. 

The new remainder above mentioned will also represent 
the date of the first Sunday in the January of the year. 



CHAPTER III. 
THE SPECTBOSCOPE. 

The Spectroscope, in producing the science of Solar 
or Celestial Physics, has in reality given us a New 
Astronomy. In 1867 Kirchhoff and Bunsen published 
their researches on Spectrum Analysis, and the birth of 
the new science is dated from that year. The Old As- 
tronomy dealt principally with the measures of celestial 
distances and magnitudes. The New Astronomy is a 
celestial chemistry, and concerns itself with the consti- 
tution of the heavenly bodies. 

A beam of sunlight is composite, and, though itself 
colorless, is really a blending together in a certain pro- 
portion of all colors. The tiny globes of water sus- 
pended in the clouds decompose the sunbeams entering 
them at certain angles, and produce the many-colored 
rainbow. These little globes separate the rays one 
from another by bending them in different degrees as 
they pass through. A glass prism acts similarly on 
sunlight, and divides it into red, orange, yellow, green, 
blue, indigo, and violet. 

When a ray of light passes through a glass prism it 
is bent from its course, and emerges in a direction differ- 
ent from the primitive one. If all the rays were equally 
bent, the light, although taking a new path, would 
come out as it went in, and there would be no analysis. 
But the rays are unequally bent, and, as a result, come 
out separated. This property of the prism is called 
refraction. The band of colors spread out on a screen 
behind the prism by a substance's light is called its 



The Spectroscope. 



41 



spectrum. By the spectrum of any object is, then, 
meant the combination of colors found in the light 
which emanates from that object. 

The rays of light passing from a rare to a more dense 
medium are all bent towards the perpendicular. The 
bending of some of the rays, however, is greater than 
that of others. The red ray is bent much less than the 
violet. Eed is at one end of the spectrum and violet at 
the other ; and between these a great variety of tints, 
caused by the interlacing of the so-called primary colors. 
The red, being bent the least, is said to be the least re- 
frangible. The violet is bent the most, and has the 
greatest refrangibility. After refraction the sunbeam 
is no longer white, but a band of many colors all the 
way from red to violet. 




Fig. 2.— Decomposition of Light by Prism. 



In this way light from any source whatever can be 
decomposed into its component parts. The character of 
the light will make known to us the constitution of the 
body whence it is emitted. In many cases the color 
alone of the light will suffice to do this. Thus a well- 
known splendid red light is characteristic of the metal 
strontium. Sodium has a yellow light, easily distin- 



4U Astkoxomy: Net? and Old. 

guishable. It is seen in the burning of common salt. 
The brilliant bluish white of the magnesium light is 
well known. 

The Spectroscope is an improvement upon the prism. 
The Spectroscope consists mainly of a small telescope, 
called the collimator, to which is attached a prism of 
flint-glass, and to the prism is attached a second small 
telescope. The collimator collects the rays from the 
object to l>e analyzed ; the prisin disperses the rays ; 
and the second telescope, being at an angle to the 
prism corresponding to the angle of the bend given to 
the rays by the prism, brings the different rays to 
their proper foci. The collimator carries a slit, to in- 
sure the admission of only a narrow beam of the light 
to be analyzed. 

When sunlight passes through the Spectroscope its 
spectrum is found not to be continuous from end to end, 
but, on the contrary, broken up or shaded over by a 
number of dark lines or divisions. These dark lines 
are a permanent feature of the solar spectrum, and 
with a strong Spectroscope run up into the thousands. 
They are of all degrees of distinctness and feeble- 
ness. 

In the solar spectrum let us take one line, the 
sodium, or, as it is marked, the D line, for instance. 
This 1) line consists in reality of two lines separated 
by a very small interval, and one of the lines is a 
little darker than the other. When the sunlight passes; 
through the Spectroscope, among the many dark lines 
of the solar spectrum the D lines are easil} 7 recognized. 
If, however, the sunlight, before entering the Spectro- 
scope, is first passed through the flame of a spirit-lamp 
colored with common salt, it will be noticed that these 
two lines composing the I) line will appear suddenly 



THE SUN. 




The Spectroscope. 43 

more vividly black, This is due to the sodiurri light 
of salt. If 110 w all light but the sodium light be re- 
moved, the prismatic colors will uo longer appear. We 
will find two bright yellow lines in the whole spectrum, 
filling exactly the places held before by the two dark 
lines of D. This is really the sodium line. The 
sodium flame produces two bright yellow lines when its 
light alone is used j but when the sunlight passes 
through the sodium flame it eliminates the yellow lines, 
and produces in their stead black lines. The sodium 
light intensifies the black lines, because the light of a 
metal passing again through its own light is absorbed. 
The sodium light from the sun, in passing through this 
sodium light of the lamp, is absorbed, and leaves only 
dark bands. This process is called selective absorp- 
tion. Here we see the very principle of spectrum 
analysis. Sodium light passing through heated sodium 
vapor is absorbed or eliminated, and where its char- 
acteristic lines should be seen in the spectrum we find 
only dark lines. 

Kirchhoff, by reversing the spectra of certain sub- 
stances, discovered that vapors of metals and gases 
absorb those rays which the same metals and gases 
themselves emit. The dark lines in the solar spectrum 
are there because something is at work cutting out 
those rays of light which are wanting. Around the sun 
and stars are absorbing atmospheres containing the 
vapors of certain elements found in the interior of the 
sun or stars. If sodium is present in the sun and is 
also present in its atmosphere, instead of finding the 
lines in the spectrum corresponding to sodium we find 
dark lines, because the sodium light of the sun's atmos- 
phere absorbed the sodium light from the sun. If iron 
is present in the sun and present also in its atmosphere, 



44 Astronomy: New axd Old. 

instead of finding the iron lines in the spectrum we see 
dark lines ; and so on of other elements. 

If we so employ a prism that while a sunbeam is 
decomposed by its upper portion a beam proceeding 
from such a light-source as sodium or other element 
may be decomposed by its lower one, we will find that 
when the bright lines of which the spectrum of the 
metal consists flash before onr eyes they will occupy 
precisely the same positions in the lower spectrum as 
some of the dark bands do in the upper solar one. So 
that each element's spectrum, besides its characteristic 
lines and color, has an established place. 

What wc can see of the solar spectrum is really but 
a fraction of the whole. Beyond the red end there are 
invisible rays of high caloric powers ; and beyond the 
violet there are invisible rays recognized by their 
chemical effects, as in photography. The invisible rays 
can be rendered visible by the agency of fluorescent 
bodies. 

Wollaston (1802) was the first to discover that the 
solar spectrum is not continuous, but crossed by dark 
bands. These bands were first, however, measured by 
Fraimhofer (1815), and are therefore usually called 
Fraunhofer's lines. It was Fraimhofer also that sug- 
gested the employment of a telescope to replace the 
screen. Fraimhofer's telescope multiplied Wollaston's 
five dark lines into six hundred. Brewster (1832), with 
a still more perfect contrivance, counted two thousand 
lines in the solar spectrum. Fraimhofer, using candle, 
gas, or lime light, perceived no dark lines in their 
spectra. Brewster was the first to offer an explanation 
of the presence of the dark lines in the solar spectrum. 
In passing light through nitrous acid gas he noticed 
that its spectrum was interrupted by dark divisions. 



The Spectroscope. 45 

The application of lieat to the gas increased the number 
of divisions, and at a very high temperature he found 
this gas opaque to sunlight. Hence he concluded that 
the presence of the dark bands in the solar spectrum 
is caused by some medium between us and the sun 
absorbing certain of the rays. Fraunhofer next made 
his experiments with common salt in the flame of a 
spirit-lamp, and saw that sunlight, passing through, 
darkened the D or sodium lines. 

Talbot and Herschel (1826), experimenting with 
glowing Lithia and other elements, showed that their 
flames rendered some of the dark bands of the solar 
spectrum still more vividly dark. They applied this 
new analysis to the detection of metals in combination, 
and so began spectrum analysis. 

Foucault (1849) by an exj)eriment illustrated the true 
theory of this new analysis, but failed to generalize the 
result. Placing salt in the electric arc, he saw the 
double bright yellow line of sodium. When he passed 
sunlight through the sodium flame the yellow lines 
disappeared and the dark lines of D were strengthened. 

Professor Stokes (1850) divined the true explanation 
of Foucault's experiment, his theory being that an 
absorbing atmosphere of sodium surrounds the sun, the 
sodium light of the atmosphere absorbing the sodium 
light of the sun. He neither verified nor published his 
theory, however, and contented himself with confiding 
it to a friend. 

Balfour Stewart, Angstrom, and other philosophers, 
ten years later, seem to have rediscovered this theory of 
Stokes. Kirchhoff, by reversing the spectra of certain 
substances, finally established the theory of absorption 
by a full and satisfactory verification, and in conjunction 
with Bunsen discovered, through the medium of the 
Spectroscope, two new metals, Oa3sium and Rubidium, 



CHAPTER IV. 

THE SUN. 

The recent progress in Astronomy has been prin- 
cipally in the direction of the Sun. This is owing to 
the improvements in the spectroscope and the great 
advances in celestial photography. What, then, does 
Astronomy teach us regarding the peerless orb of day? 

The diameter of the Sun is 800,000 miles. If the 
Sun could bedivided into a million parts, each one of 
them would greatly exceed the earth in bulk; and the 
Sun weighs more than 326,000 earths. The mass of 
the Sun is more than 745 times the united masses of 
all the other bodies within the solar system. 

As matter attracts in proportion to its mass, it is 
his great weight that enables the Sun to hold in their 
places the planetary bodies that he governs, and pre 
vent them from rushing away wildly into space. All 
our planetary bodies are moving very rapidly, and, 
according to the first law of motion, would, unless pre- 
vented by the Sun, sweep on for ever in straight lines 
through the mighty realms of space. 

The Sun is intensely hot. Its temperature far ex- 
ceeds any artificial heat with which we are acquainted. 
It is much above that of the electric arc. When we 
get to the top of a lofty mountain we are nearer to 
the sun than when we are on the surface below. Still 
it is colder on the mountain's top than below. This 
is seemingly a paradox, as the nearer we are to a heated 
body the more of its warmth we should feel. The rea- 
son of the mountain's cold is this: The air lets the 

40 



The Suit. 47 

heat of the Sun pass through it without retaining' any. 
Air is diathermanous, and is heated by convection. The 
layer of air in immediate contact with the earth's sur- 
face is heated first, and being thereby rendered lighter, 
it ascends. The colder layer above descends to replace 
the first layer, and being in turn heated, it too ascends 5 
and so on. 

. On a summer day it is warmer inside a green-house 
than outside, because the glass, while permitting the 
heat to pass in, will not let it pass out so readily. 
The earth resembles a green-house, where the air re- 
places the glass panes. If we had no atmosphere, the 
earth's surface, like the mountain tops, would be covered 
with eternal frosts and snows. 

The Sun appears to the unaided eye as a flat lu- 
minous circle of dazzling brightness, and absolutely 
without a blemish. When viewed through a telescope 
with a protected eye-piece the Sun has the appearance 
of a glowing globe with a mottled or granular sur- 
face. 

The Sun consists of a nucleus, or interior globe, and 
three envelopes. The first envelope is the Photosphere; 
the second the Chromosphere, or Sun's true atmosphere; 
and the third the Corona. 

The Photosphere.— The bright granular surface seen 
through the telescope is the photosphere, or, as its name 
implies, the sphere of light. To most observers this 
visible disc appears as if spread over with rice-grains, 
but an English astronomer, Nasmyth, claimed that it 
resembles a surface strewn with bright willow-leaves. 

When the Sun is viewed with a telescope of even 
moderate size its bright granular surface is seen to be 
pitted with black spots. These spots are occasionally 
very numerous. They vary in number and size from 



4s Astronoxiy: New and Old. 

time to time. They are observed to have a regular 
constant motion across the Sun's face. 

From the common motion of all the spots in the same 
direction, it is concluded that the Sun turns on its axis 
in an interval of about 25 or 2G days. An extraordinary 




Fig. 4.— Su> T Spots. 



feature of the Sun's rotation is that it does not turn 
like the earth all-of-a-piece. but some parts move faster, 
and, indeed, make more turns, than others. This is a 
strong argument against the Sun's solidity. The equa- 



The Sex. 49 



torial regions revolve in a shorter time than do the 
higher latitudes. 

Carrington computed the equatorial period at a little 
less than twenty-live days, and that of latitudes 50° at 
twenty-seven and a half. 

The spots usually present a dark central part, 
strongly contrasted with the brighter margin. The 
dark centre is called the umbra or nucleus. The um- 
bra is surrounded by a border, not so dark as the 
centre nor so bright as the disc. This border region is 
called the penumbra. It appears ordinarily of a uniform 
grayish tint. This penumbra, when highly magnified, 
appears streaked with radiations pointing towards the 
spots' centre. The spots are mostly found in groups. 
A large spot frequently breaks up into a number of 
smaller ones. A spot may live only a few days, and 
may survive several months. 

Faculse. — The surface of the Sun is not uniformly 
bright. It is brightest at the centre and the lustre 
gradually fades as we approach the edge. We fre- 
quently see scattered over the Sun's disc luminous 
patches or tongues, called faculse, much brighter than 
the surrounding regions. Although appearing as bright 
fissures in the Sun, the faculae are in reality above the 
solar surface. They are most apparent towards the 
Sun's rim, because of the comparative darkness of these 
outer regions. The darkening of the Sun's face toward 
the edge is caused by its atmosphere. The rays of light 
emitted from the vicinity of the edge have to travel 
through a greater depth of atmosphere than those from 
the central regions. Our own atmosphere acts similarly 
on the light rays reaching us. As we gaze upon the 
Sun when overhead, we see it through a less amount of 
air than when we view it upon the horizon. And so 



50 Astroxomy : New axd Old. 



it is brighter id the zenith than when rising or setting. 

It* the Sun had no atmosphere it would be about twice 
as bright and hot as it is, and its color much bluer. 

According to Professor Langley, the photosphere is 
pnrely vaporons, and its mottled aspect is due to the 
appearance of numerous faint dots on a white back- 
ground of fleecy clouds resembling our cirri. By using 
high magnifying powers these clouds may be divided 
ii]) into numbers of small, intensely bright bodies float- 
ing apparently in a dark medium. The dark openings 
between these small bright floating bodies are called 
pores. These pores are of different sizes. The small 
bright bodies may be again broken up into still smaller 
bright particles. The photosphere would thus appear 
to have three orders of brightness: the cloud-like 
appearances; the rice-grains into which these clouds 
can be resolved ; and a still liner division of small bright 
grannies composing the rice-grains. Langley considers 
that the mottled appearance of the photospheric clouds 
is caused by a vertical circulation of currents, absorb- 
ing into the Sun's interior the cold matter from without, 
and sending forth heated matter from within towards 
the surface. 

Spots. — In 1(110 Galileo and Fabricius first discovered 
the existence of sun-spots. Galileo considered them 
solar clouds, and was the father of the cloud theory. 
Simon Marius was the author of the slag theory, and 
regarded them as the cindery refuse of the solar 
combustion. Derham (1703) was of the opinion that 
they are solar volcanoes. Lalande looked upon them 
as rocky elevations. Wilson (1774) regarded them as 
vast excavations in the Sun's substance. Sir John 
Herschel (1837) noticed that the spots occupy two zones 
on each side of the Sun's equator. These zones, or belts, 



The Sum 51 

extend from the 6th to the 35th degree of latitude on 
botli sides of the solar equator. 

Carrington was the first investigator of the drift 
of spots across the Sun's face. Sir John Herschel 
suggested the cyclonic theory of sun-spots in 1847, 
and Faye, in 1872, the whirlpool theory. Both these 
theories are now generally abandoned. 

Heinrich Schwabe, of Dessau, was the first to an- 
nounce the periodicity of sun-spots. This he did in 
1843, and fixed the period of their alternate abundance 
and scarcity at ten years. Dr. John Lainont, in 1851, 
estimated the period at 10£ years. Eudolph Wolf, in 
1852, placed the period at 11.11 years. In 1859 Wolf, 
after further thought on the subject, concluded that 
although the mean period is 11.11, still there is consid- 
erable fluctuation on either side of this mean. The 
interval between two maxima may reach 16 £ years or 
descend to 7J years. It is also understood that the 
spots increase more rapidly than they decrease, and that 
neither the increase nor decrease is uniform. 

Two other sun-spot periods have also been discovered, 
one of 55 J years, and one of 222 years. 

There is a close connection between sun-spot periods 
and magnetic changes upon the earth. Humboldt no- 
ticed that magnetic " storms" attained their greatest 
violence every ten years, and Sabine announced the 
correspondence between the sun-spot periods and the 
ebb and flow of magnetic change. 

There is a feeble connection between sun-spot periods 
and the lustre of certain variable stars. An endeavor 
was early made to connect spot periods with the weath- 
er. William Herschel made the first attempt of this 
kind. He thought that a great emission of light and 
heat accompanied an increased number of spots. 



52 Astronomy: New and Old. 

Schwabe studied tlie matter carefully, but failed to 
discover any connection between tlie spots and weather 
changes. Wolf, iu 1859, announced that he could detect 
no connection between these phenomena. This question 
of tlie influence of spots on the weather has still, how- 
ever, received no satisfactory answer. There would 
seem to be some grounds tor the opinion that increased 
rainfall attends the maxima of spots. 

The Aurora Borealis is intimately associated with 
solar disturbance. The aurora borealis, sun-spots, and 
magnetic activity rise and fall so closely together that 
they must depend upon ;i common cause. 

The spectrum of sun-spots, with some minor differ- 
ence, is iiie same as thai of the Sun. The same vapors 
are found in the sun spot cavity as in the unbroken 
solar surface. The vapors in and about tlie spots often 
move, by the testimony of the spectroscope, Avith a 
velocity as high as 320 miles a second. The temperature 
Of the spots is lower than that of the photosphere. 

There is a suspicion that the planetary movements 
have a slight influence on sun-spots. There is, however, 
nothing definite in this respect. 

.Many of these spots cover areas on the Sun of vast 
proportions. Some of the great ones have measured 
over a billion square miles, or more than five times 
the terrestrial surface. The cavities of the spots are 
very dec]). We can gaze down into some of them to a 
depth probably of five thousand miles. As we look 
down into the vast depths, we see only volumes of cir- 
cling vapors, with no evidence of either solid or liquid 
groundwork. 

The photosphere is a surface of condensation, being 
the limit set by the cold of space to the Sun's internal 
process of heat convection. Every cooling cosmic body 



The Sum 53 

has a surface so defined. The stores of heat of the Sim's 
interior reach the surface by means of vertical convec- 
tion-currents. The photospheric clouds may probably 
be composed of carbon, silicon, and boron. The 
u grains/' or bright parts of the photosphere, represent 
the upper terminations of the ascending warm currents, 
and the descending cooler currents produce the darker 
parts, or pores. The grains are computed by Langley 
to compose one-fifth of the photospheric surface, and 
to furnish three-quarters of the Sun's light. Janssen 




Fig. 5.— Sun-Spot (Secchi). 

calculates that if the whole surface of the Sun were 
uniformly as bright as these grains, it would emit from 
ten to twenty times its present amount of light. The 
photosphere is the seat of the Sun's light and heat. 

Owing to the awful brightness of the photosphere, 
the two outer solar envelopes, the chromosphere and 
corona, cannot ordinarily be seen through the telescope. 
These solar surroundings are visible only during a total 
eclipse of the Sun, or through a spectroscope. 



54 Astronomy: New and Old. 

Solar Eclipses. — A solar eclipse is caused by the pas- 
sage of the moon's dark body between the earth and 
Sun. The moon, in its translatory motion around the 
earth, frequently passes between us and the Sun. The 
solar eclipse always takes place at new moon. The 
moon is then in conjunction, or almost in a line between 
the earth and Sun. If the path of the moon were 
exactly on the ecliptic or apparent path of the Sun. 
there would be a solar eclipse at every new moon. But 
the path of* the moon is inclined to the Sun's path, 01 
the ecliptic, by an angle of five degrees and eight 
minutes. 

The plane of tin- moon's path being thus inclined 
to the plane of the ecliptic by this angle, it necessarily 
crosses the plane of the ecliptic at two points. Half 
of this plane is above, and the other half below, the 
plane of the ecliptic. The moon makes a revolution of 
its orbit in 29J days which is the length of a lunation. 
The moon is half a lunation above the plane of the 
ecliptic, and the other half below it. The two points 
where the plane of the moon's path intersects the eclip- 
tic are called the moon's nodes. 

Where the moon crosses to go north or above the 
ecliptic is called the ascending node, and where it 
crosses to go south or below, the descending node. 

If the moon's path were less inclined to the ecliptic 
there would be more eclipses in any given number of 
years than now take place. If the moon's path were 
more inclined to the ecliptic than it now is, there would 
be fewer eclipses. 

The time of the year in which eclipses happen 
depends on the position of the moon's nodes on the 
ecliptic, and if that position were always the same, the 
eclipses would always happen in the same months of 



The Sum 55 

the year. Owing to the effect of the Sun's attraction 
upon the moon, the lunar nodes move backward at the 
rate of 19 degrees and 19 minutes per year 5 and it 
takes the Sun 20 days to travel that distance, so that 
the eclipses must take place 20 days earlier each year, 
or at intervals of about 316 days. 

In a period of 223 lunations the nodes return to the 
same place. This period of 223 lunations requires 6,585 
days, 7 hours, and 12 minutes, or about 18 years and 11 
days. Therefore, after a lapse of 18 years and 11 
days from any eclipse, we shall return to a similar 
eclipse 5 and it will be an eclipse of about the same 
magnitude as the one from which we reckon. This 
X^eriod was discovered by the Chaldean astronom- 
ers three thousand years ago, and they were thus 
enabled to calculate to within less than one day of the 
true time of an eclipse. Modern astronomers, by a 
more elaborate method, can compute to the precise 
second the approach of an eclipse. 

The moon is a much smaller body than the Sun, but 
owing to the Sun's greater distance from us, the appar- 
ent size of the moon is generally equal to the Sun's 
apparent size. The path of the earth in its translatory 
motion around the Sun is an ellipse. The Sun is 
located at one of the foci of this ellipse. At the winter 
solstice the Sun is much nearer to us than at the sum- 
mer solstice. This variety of distance increases or 
diminishes the Sun's apparent magnitude, so that some- 
times it is greater, sometimes less, and sometimes equal 
to the moon's apparent magnitude. 

The moon's orbit is also elliptical, and our satellite 

is sometimes nearer to us than at other times, and so 

its apparent magnitude varies a little from time to time. 

When the apparent size of the moon is greater than 



56 - 1 s tr o NOM 1 ' : Ne i v a xn Old. 

the apparent size of the Sun, the moon cuts off the sun- 
1 in lit completely from a portion of the earth, and 
renders that portion almost as dark as night. But why 
a portion, and not the whole illuminated half of the 
earth I The moon is a much smaller body than the 
earth, the moon's diameter being only a quarter of the 
earth's, so that at its best the shadow of the moon could 
cover only the one-thirteenth part of the earth's face. 
Besides, the shadow of the moon is shaped like a cone. 
The volume of the Sun is nearly thirteen hundred 
thousand times the volume of the earth, and the volume 





Fig. 6.— Total Solar Eclipse. 

of the moon is only the one-forty-ninth of the earth's 
volume. Lines drawn touching; the surfaces of the two 
spheroids of the Sun and moon, and produced until they 
meet behind the moon, will form the cone of shade cast 
by the moon in the earth's direction. 

Sometimes, owing- to the relative distances of the 
Sun and moon from the earth, the apex of this cone 
of shade just reaches the earth, and sometimes it does 
not. When the apex of the shadow reaches the earth 
and laps a little over, as it rarely does when favored by 
the small changes in the distances of the Sun and moon, 



The Sum 57 

at that place on the earth's surface touched by the 
shade there Trill be a total eclipse. TThen the shadow 
does not reach the earth there is only an annular 
eclipse. 

There are three kinds of solar eclipses : partial, 
annular, and total. In a partial eclipse the moon does 
not pass directly between the earth and Sun, but slips 
by a little on one side, cutting off from view only a por- 
tion of the solar disc. In an annular eclipse, although 
the moon passes directly between us and the Sun, the 
apex of the lunar shadow does not reach as far as the 
earth, and there consequently appears around the 
moon's dark body a ring or annulus of light. In a total 
eclipse the Sun entirely disappears behind the moon's 
dark body. 

The space embraced by a total eclipse is very small. 
The shadow appears as a dark circle whose diameter 
is usually about fifty miles. The diameter of this circle 
of shade scarcely ever exceeds 150 miles. But this 
black circle runs along after the moon, and in that way 
darkens a long, narrow strip of the earth's surface. 

The duration of an eclipse is variable. We must 
distinguish, however, between its duration as regards 
the whole earth and one given place. The greatest 
possible duration of an eclipse at the equator is 4 hours, 
29 minutes, and 44 seconds. The greatest possible 
duration of total obscurity at one given place on the 
equator is 7 minutes and 58 seconds. 

Total solar eclipses are rare for the earth in general, 
and exceedingly so for any particular place, owing to 
the narrowness of the limits embraced. 

The Nautical Almanac x>ublishes, years in advance, 
all the indications of an eclipse. The exact time and 
the dimensions or phases of the eclipse are given for 



58 4 1 s tr oxo my : Xe w a xd Old. 

every spot of the earth. The size of the phases of an 
eclipse is expressed in almanacs in digits ; a digit being 
the twelfth part of the diameter of the disc of the sun. 

During the period of totality the sensation of awe 
is supreme. It grows so dark that the stars of the 
second magnitude appear in the sky, as do also the 
planets of the first order of brightness. There is a 
very perceptible lowering of the temperature. Animals 
and plants are unmistakably affected. Convolvuli, 
poppies, and night-shades have been seen to half open 
from being previously entirely closed. 

Professor Langley describes the eclipse of 1869, 
which he viewed from a position near the Mammoth 
Gave in Kentucky, as follows: " First the black body 
of the moon advanced slowly on the sun, as we have 
all seen it do in partial eclipses, without anything 
noticeable appearing; nor till the sun was very nearly 
covered did the light of day about us seem much 
diminished. But when the sun's face was reduced to 
a very narrow crescent the change was sudden and 
startling, for the light which fell on us not only 
dwindled rapidly, but became of a kind unknown 
before, so that a pallid appearance overspread the face 
of the earth with an ugly livid hue; and as this strange 
wanness increased, a cold seemed to come with it. The 
impression was of something unnatural ; but there was 
only a moment to note it, for the sun went out as sud- 
denly as a blown-out gas-jet, and I became as suddenly 
aware that all around where it had been there had 
been growing into vision a kind of ghostly radiance, 
composed of separate pearly beams, looking distinct 
each from each, as though the black circle where the 
sun once was bristled with pale streamers, stretching 
far away from it in a sort of crown." 



The Sun. 



59 




00 Astronomy : New axd Old. 

Chromosphere— During totality great tongues of 
flame, of a vivid crimson and scarlet color, are seen 
spreading out in various directions from the moon's 
limbs. These protuberances or prominences have been 
examined by the spectroscope, and found to be prin- 
cipally composed of hydrogen gas in a highly heated 
condition. These tongues of flame are eruptions of the 
sun's atmosphere. This atmosphere, called by Lockyer 
the chromosphere, is a thin envelope, composed prin- 
cipally of hydrogen, surrounding the whole surface 
of the sun. 

Young, speaking of this chromosphere, says that "the 
appearance is as if countless jets of heated gas were 
issuing through vents and spiracles over the whole sur- 
face, thus clothing it with flame, which heaves and tosses 
like the blaze of a conflagration." 

These tongues or prominences reach out to vast 
distances, some as far as 50,000 miles beyond the Sun's 
surface. 

The spectroscope is now so perfect, however, that 
in its analysis of these names it can dispense with an 
eclipse. 

The Corona.— During totality, and just immediately 
before and after, a luminous halo surrounds the disc of 
the Sun. It is the corona, and it reaches out to an enor- 
mous distance, all the way from 300,000 to 1,000,000 
miles from the Sun's surface. Occasionally it has been 
seen to stretch away into space as far as ten and even 
fourteen million miles. It is of almost indefinite tenuity, 
and has no counterpart in the material composition of 
our planet. Its color varies all the way from pearly 
white to red. The most striking feature of its spectrum 
is a single bright green line, which corresponds to no 
known element upon the earth. 



The Sum 61 

Professor Hastings thinks that the corona is pro- 
duced by diffraction upon the moon's edge. The pheno- 
menon of diffraction may he seen in the early morning, 
when the rising Sun fringes with a line of light objects 
on the horizon. 

The corona is not a solar atmosphere. It is, however, 
a solar appendage, and may be the resultant of the ac- 
tion of electrical repulsion, in one direction, and of 
gravity, in the other, upon some very attenuated sub- 
stance. It does not gravitate upon the Sun's surface 
nor share in his rotation. 

The spectroscope would seem to make it a compound, 
partly of a self-luminous gas, principally hydrogen, and 
the unknown substance of the green line $ and partly 
of solid or liquid particles reflecting sunlight. 

Solar Constitution. — The spectroscope has detected 
the presence in the Sun of the following elements : 

Hydrogen, Aluminium, Mckel, Strontium, 
Sodium, Iron, Zinc, Cerium, 

Barium, Manganese, Copper, Uranium, 
Calcium, Chromium, Titanium, Lead, 
Magnesium, Cobalt, Cadmium, Potassium. 

Sun's Distance. — To find the correct distance to the 
Sun is regarded as the most important problem in 
Astronomy. In all celestial measurements beyond the 
moon the Sun's distance is the unit, and an insigni- 
ficant error in this unit would become an enormous one 
when the nearest fixed star is reached. 

It is also of moment to know this distance correctly 
when we undertake to calculate the Sun's temperature. 
For the comparative intensity of the solar rays at the 
earth's surface and at the Sun's surface is governed 
by this distance. 



62 Astronomy: New and Old. 

To compute the Sun's distance we must find its hori- 
zontal parallax. This is the angle subtended by the 
earth's radius at the Sun's distance. It is the magnitude 
of the earth's semi-diameter as seen from the Sun. 
This parallax, or angle, cannot be found directly by the 
finest instruments ; not so much on account of its small- 
ness as because of atmospheric troubles. Air refraction 
near the horizon prevents the proper measurement of 
SO small an angle. 

Astronomers have solved the problem by indirect 
methods. The relative distances of the planets from 
the Sun are governed by an inflexible law — Kepler's 
Third Law; or that the square of the time of revolution 
of each planet is proportional to the cube of its mean 
distance from the Sun. If we can find the correct dis- 
tance of a single planet from the earth, the problem is 
solved. Mars is sometimes (after intervals of fifteen 
years) within thirty-five million miles of us. The first 
scientific estimate of the Sun's distance was made 
through Mars. Richer and Gassini made it in 1072. 
They computed the parallax at 9.5", which corresponds 
to eighty-seven millions of miles. 

Flamstoed, by the same method, made the parallax 
10", or 81,700,000 miles. Picard made the distance 
41,000,000, and Lahire 136,000,000, miles. 

Venus sometimes approaches to within 2G,000,000 
miles of us. This occurs when she passes between the 
earth and Sun, or during a transit. These transits hap- 
pen in pairs at intervals of 105 J and 121 J years. The 
pairs are separated from each other by eight years, less 
two and a half days. The rarity of the transits is due 
to the large angle which the plane of the orbit of Venus 
makes with the Sun's path. 

As the result obtained through the transits of 



The Sux. 63 

1761 and 1769, Encke gave us the classic number of 
95,000,000 niiles. He placed the parallax at 8.5776", 
or 95,250,000 ; in round numbers, 95,000,000 miles. 

Hansen, in 1851, and Leverrier, in 1858, declared that 
this distance would not accord with the motions of the 
moon ; that it was too great by nearly 4,000,000 miles. 
Xewcomb, in 1865, by taking Mars instead of Venus, 
made the figures 92,500,000. 

The velocity of light affords another means of finding 
the distance to the Sun. Delambre, in 1792, from 
observations on the eclipses of Jupiter's satellites, com- 
puted that the time required for light to travel to us 
from the Sun is 193.2 seconds. Glasenap, a Eussian 
astronomer, in 1871, made it 500.84 seconds. 

Fizeau, Wheat-stone, and Leon Foucault measured 
the velocity of light by means of revolving mirrors. 
Foucault announced, in 1862, that, according to this 
method, the Sun must be considered much nearer than 
by Encke's computation. 

Elaborate preparations were made for observing the 
transit of Yenus of December 8, 1874. It was hoped 
that great precision would be reached. But this hope 
proved futile, as the distorting effects of refraction, 
caused by the atmosphere of Venus, marred, in a great 
measure, the accuracy of the observations, There was 
a great want of unanimity in the results reached by 
astronomers. 

Sir George Airy made the parallax 8.754", corre- 
sponding to 93,375,000 miles. Stone obtained a value 
of 8.88", or 92,000,000. From photography Mr. Todd 
deduced a parallax of 8.883", or about 92,000,000 
miles. 

Skilful preparations were made for observing the 
transit of December 6, 1882. But this transit, like- 



64 Astronomy: New and Old. 



wise, failed to furnish a satisfactory solution of the great 
problem, and the range of doubt remained as wide as 
before, the atmospheres of the Sun, the earth, and Venus 
still combining to defeat precise observation. The fol- 
lowing is a list of the Sun's distances from us as com- 
puted by eight great authorities: 

Newcomb, 92,500,000. Young, . 92,885,000. 

Dr. Gill, . 93,080,000. Faye, . . 92,750,000. 

Todd, . . . 92,800,000. Harkness, 92,365,000. 

Ball, . . . 92,700,000. Stone, . . 92,000,000. 

Sun's Heat. — The heat of the Sun is indeed enor- 
mous. It is computed all the way from eighteen thou- 
sand to beyond a million degrees Fahrenheit. Accord- 
ing to the law of decrease of radiant heat, the heat of 
the Sun at its own surface is 300,000 times more intense 
than at the surface of the earth. If the Sun were to 
come as near to us as the moon, the solid earth itself 
would not only melt like wax, bu£ be dissipated into 
vapor. 

The heat received on the half of the earth turned 
towards the Sun is but the two-thousand-millionth part 
of the entire amount radiated into space. 

Effects of this awful Heat — Owing to this enormous 
heat, the surface of the Sun is in a constant state of tur- 
moil. Hurricanes and fierce cyclonic storms are inces- 
santly sweeping over it with velocities exceeding a 
hundred miles a second. The unceasing tumult and 
uproar on every part of the solar surface are great 
beyond expression. 

This heat also occasions mighty eruptions and ex- 
plosions, by which matter is expelled to distances often 
of 160,000 miles with an initial velocity of 500 miles 
a second. 



The Sun. 65 

These vast eruptions produce cavities in the solar 
surface, and, in Secchi's hypothesis, are the origin of 
sun-spots. 

Source of Solar Heat. — The Sun is constantly radiat- 
ing its heat into space, and it would certainly grow 
cold unless some source supplied the loss. Among the 
many hypotheses purporting to account for this heat- 
supply, the gravity one appears the most circumstantial. 
According to this hypothesis, the Sun's own force of 
gravity causes a contraction of his diameter. It is a law 
of mechanics that contracting gas generates heat. The 
Sun is gaseous, but of great viscosity. A yearly shorten- 
ing of the solar diameter of about 220 feet would be am- 
ply sufficient to supply the heat lost through radiation. 
This yearly contraction would be inappreciable at the 
Sun's great distance. 

Helmholz first announced this hypothesis in 1854. It 
is the single pivot on which Laplace's Nebular Hypo- 
thesis now rests. Helmholz's hypothesis is strenuously 
opposed by geologists and biologists, as not allowing 
them sufficient time lor their own hypotheses. 

The Sun is the source of our light and heat, and, 
indeed, of all our comforts. Its bright rays gladden the 
day, and its reflected beams impart to the night its 
softer glory. Its chemical rays are necessary for the 
growth of all animal and vegetable life on the earth. 
This great star is the primary source of our clouds, our 
rain, our breezes, our stores of coal, and of the gaslight 
in our cities. To us it is truly the almoner of the 
Almighty. 



CHAPTER V. 

THE MOON. 

The Moon, relatively to us, is the heavenly body, 
after the sun, of most importance. It is our nearest 
neighbor in the realms of ether. It is the ever-faithful 
companion of the earth, and accompanies it in its yearly 
journeys around the sun, and in its mighty pilgrimage 
through space. 

On account of its proximity, the Moon has been 
studied more than any other of the heavenly host, 
and, consequently, we are very familiar with its promi- 
nent features; and its laws, motions, and history are 
known with accuracy. We will first consider the fea- 
tures which our satellite presents to the unaided eye. 

A Lunation. — One of the first things noticed about 
the Moon was its eastward motion among the stars. If, 
on a clear night, we observe that the Moon is close to a 
bright star, and watch both Moon and star for a few 
hours, we will perceive that the Moon has moved per- 
ceptibly away towards the east from the star. If we 
observe them at the same hour on the following night 
we will find that the Moon has moved towards the east 
about 13 degrees of an arc, corresponding to 50 minutes 
of time, in the interval of 24 hours. It makes a circuit 
of the heavens in 294 days. The Moon travels from one 
position in the heavens back to the same position, rela- 
tively to the sun and earth, in 29J days. This is the 
length of a lunation, or lunar month. 

Moon's Phases. — As the travelling Moon thus changes 
its positions it presents different aspects or phases. It 
is new, in the first or last quarter, and full. 



The Mo ox. 67 



When the Moon is full it is on the opposite side of 
the earth from the sun, and shows its full illuminated 
disc to the sun and earth together. Moving onward 




Fig. 8.— Phases of the Moon. 
in its course, it gradually passes more and more between 
the earth and sun, getting in its own shadow as far as 
concerns us, and the portion of its surface reflecting 



68 Astronomy: New and Old. 

to us the sunlight grows smaller and smaller. Thus, 
after being full, it rises later each night, and each night 
the size of its enlightened disc grows less. About 
a week after the full but half its face is illuminated, the 
other half being dark ; it is then in the third or last 
quarter. After this it presents a crescent form, until, 
in the space of another week, it disappears entirely; 
being on the same side of the earth that the sun is, 
it is swallowed up in the awful brightness of the sun's 
beams, and is completely invisible for about four days. 
After this it floats away from the sun and appears very 
low in the extreme west just after sunset. It then pre- 
sents a bright crescent, very slender, and the convexity 
towards the sun, as is natural, since its light is borrowed 
from that luminary. 

The crescent thence grows from night to night for 
the space 1 of a week, when, having receded 90° from the 
sun, half the Moon's disc is illuminated; it is then 
in the first quarter. After this the disc becomes more 
and more illuminated, the phase increases in size until 
the Moon becomes full again ; and so the phases succeed 
each other as before. Thus does the Moon constantly 
go through the same successive phases, presenting new 
Moon, first quarter, full Moon, and last quarter. 

Earthshine. — When the Moon is very new its dark 
part is faintly illuminated and becomes visible to us. 
This feeble illumination of the dark part is known as 
earthshine, and is the light of the earth reflected back 
to us from the Moon's night side. The earth's disc is 
13. J times that of the Moon, and, as a consequence, the 
earth shines on the Moon with 13£ times the intensity 
that our satellite shines on us. 

The color of the moonlight is yellow, while the color 
of this earthshine is ash-gray. With a powerful tele- 



The Mo ox. 



scope mountain peaks can be distinguished in this ash- 
gray region. The intensity of this ash-gray light varies 
according as continents, great forests, sandy deserts, or 
oceans throw the'r reflection upon the Moon. 

The ash-gray may occasionally change to olive-green. 
Lambert, an observer, relates a change of this nature as 
follows : " The Moon, which then stood vertically over 
the Atlantic Ocean, received upon its night side the 
green terrestrial light which is reflected toward her 
when the sky is clear by the forest districts of South 
America." 

Permanence of the Moon's Features. — Besides its mo- 
tion around the earth, the Moon has a motion on its 
axis. It turns on its axis in precisely the same time that 
it revolves around the earth. This coincidence is not 
due to chance. There is a projection, or bulge, on the 
axis of the Moon pointing towards the earth, known as 
the major axis, and the earth, acting on this projection 
by gravitation, continually retarded the Moon's axial 
motion, until at length the motion round the earth and 
the motion on the axis corresponded in point of time. 
In consequence of this coincidence we never see but one 
side of the Moon. A person will easily understand how 
the Moon makes a complete revolution on its axis, and 
still keeps the same side always turned towards us, by 
placing a chair in the middle of the room and walking 
around it, keeping the face constantly towards the 
chair. Though your face is always towards the chair, 
nevertheless you turn completely around in going around 
the chair, so that your face turns to all the points of 
the compass. The period of these revolutions will, in 
future, always correspond, because the force that pro- 
duced their isochronism will maintain it. 

The aforesaid bulge was produced upon the Moon by 



70 Astronomy: New and Old. 



the tidal action of the earth on the Moon's plastic mass. 
Tidal action may, then, be said to have caused this 
isochronism. 

Sidereal Revolution.— In 29 days, 12 hours, 44 minutes, 
and 3 seconds the Moon returns to occupy the same 
position with respect to the sun and earth, and this is 
called a lunation ; but the Moon makes a sidereal revo- 
lution of its orbit, or moves from a fixed star back to the 
same fixed star in 27 days, 7 hours, 43 minutes, and 11J 
seconds. A difference of more than two days is caused 
by the earth's motion of translation in its orbit at the 
same time. While the Moon is going around her orbit 
the earth travels quite a distance in her own path, and 
it takes the moon a little over two days, after finishing 
her course, to overtake the earth. 

Moon's Distance and Diameter.— Professor Adams com- 
putes the mean distance between the centres of the 
earth and Moon to be 238,793 miles. The path of the 
Moon is elliptical, or oval, in shape, and so our satellite 
is sometimes nearer to the earth than at other times. 
The greatest difference in the distances may almost 
reach thirty-nine thousand miles. In consequence of 
this change of distance, the apparent size of the Moon 
varies. Hansen gives 31' as the measure of the Moon's 
apparent diameter. The sun's apparent diameter is 32'. 
The usually received measure of the Moon's real dia- 
meter is 2,160 miles. The Moon weighs about the one- 
eightieth of the earth. 

When the Moon is in the zenith, or overhead, it is 
nearer to us by the earth's radius than when it is just 
rising or setting. Yet, by an optical illusion, it looks 
larger on the horizon than when overhead. This illusion 
is due to the eye contrasting the rising Moon with ob- 
jects on the earth in the line of vision. Looking at 
the moon through a tube the illusion disappears. 



The Mo ox. 71 



Geography of the Moon. — When viewed through the 
telescope the Moon's surface appears terribly broken up 
and mottled. Its pitted face resembles a muddy plain 
into which showers of stones had been thrown. The 
hilly parts of the Moon's broken surface reflect sun- 
light better than the flat levels, and so the mountainous 
ridges look bright and the plains dark. 

The ancients supposed the dark regions to be com- 
posed of water, and named them seas. The name of 
"seas" is still retained, although it is understood that 
there is no water on the Moon. 

One of the most striking features of the Moon's disc 
is an oval spot, of a mixed grayish and dark olive 
tint, close to the western limb. This is the Mare 
Crisium, or Sea of Crisis. It looks darker than any 
other of the so-called seas, on account of its isolated 
position, and, consequently, more striking contrast with 
the surrounding white. The Sea of Crisis contains 
12,000 square miles. 

The Sea of Tranquillity lies between Crisis and the 
Moon's centre. Tranquillity contains 23,200 square miles. 

The two seas running up from Tranquillity are 
Plenty and Nectar. The western one is Plenty, while 
Nectar is nearer to the Moon's centre. 

Reaching northwardly from Tranquillity is the Sea 
of Serenity, rendered very striking by the presence of 
a bright ray running through its entire length. 

East of this last is the Sea of Rains, containing 
64,000 square miles. 

East of the Sea of Rains is the extensive but poorly- 
defined Ocean of Tempests, with an area of about 
360,000 geographical miles. 

Far to the southward of Tempests lie the Sea of 
Humors and the Sea of Clouds. 



72 A stm o .vo.v r: Nb w a nd Old 

The spots on the Moon cover about two-fifths of the 
whole disc. 

Mountain Chains. — The most remarkable of the lunar 
mountain chains are the Alps, the Caucasian chain, and 
the Apennines. These three £»reat ranges surround, as 
by a semi-circle, the Sea of Rains. The Seas of Serenity 
and Vapors are separated from the Sea of Eains by the 
Apennines. 

The Seas of Eains and Clouds are divided from 
the Ocean of Tempests by the Carpathian and Oural 
Mountains. 

The Sea of Serenity has the Taurus Mountains on 
the west ; and the range of the Pyrenees lies between 
the Seas of Plenty and Nectar. 

The Mountains of Dorfel and Leibnitz are at the 
southern pole. Westwardly lie the Altai Mountains, 
and close to the western limb are the Cordilleras and 
D'Alembert ranges. 

The Apennines are 373 and the Altai Mountains 276 
miles long. 

Mountain Scenery. — There are four varieties of moun- 
tain scenery on the Moon : Insulated mountains, ris- 
ing from level plains and shaped like sugar-loaves ; 
mountain ranges somewhat resembling the great chains 
on the earth j circular ranges encompassing, like a 
mighty rampart) a plain or great cavity; lastly, central 
mountains, or mountains in the centre of circular plains. 

The height of the lunar mountains may be computed 
by measuring the distance of their illuminated summits 
from the enlightened part of the disc with a micrometer, 
and obtaining at the same time, by observation, the 
positions of the sun and Moon. The height of the lunar 
mountains is known as accurately as that of the chief 
mountains on the earth. 



The Mo ox. 73 



Mountains on the Moon. — Among the loftiest moun- 
tains on the Moon are a peak of the Leibnitz range, 
41,900 feet above the valley; Dorfel, 26,691 feet 5 Our- 
tius, 22,227 feet; Casatus, 21,234 feet; Calippus, 18,579 




Fig 9.— Photograph of a Lunar Phase, by Professor Charroppin, S.J., St. 
Louis University. 

feet, and Huyghens, 18,060 feet in height; and the crater 
walls of Newton, 23,853 feet deep. 

The best time to see the really broken" character 
of the lunar surface is to observe it at the first or last 



74 Astronomy: New and Old. 

quarter. Along the line of separation of the light and 
shadow, called the Terminator, the interior of the annu- 
lar cavities seems quite black, whilst at intervals lumi- 
nous points show themselves detached from the enlight- 
ened portions of the Moon. These brilliant spots are 
mountain tops, and according as they are observed at the 
first or last quarter, reflect the Moon's morning sun or 
the sunset rays which linger after the plains are envel- 
oped in shadow. 

Caverns. — Numbers of immense caverns lie all over 
the Moon's surface, Nearly one hundred of them can 
be distinguished, grouped together in the south-west 
part alone. The mouths of the caverns are mostly sur- 
rounded by mountain ridges, which reflect light very 
well, and give a bright appearance to those parts of the 
Moon where they abound. 

Lunar Volcanoes. — The number of extinct volcanoes 
on the Moon bears testimony that it must have, during 
its period of activity, suffered frightful convulsions. 

Craters. — The size of the lunar craters is enormous. 
The diameter of the crater Ptolemy is 114^ miles; Plato, 
60 miles; Copernicus, 5G miles ; Tycho, 54 miles, and of 
Aristarchus 23 miles. 

Aristarchus lies between the Sea of Bams and the 
Ocean of Tempests, and is the most brilliant spot upon 
the surface of the Moon. So bright is it, indeed, that 
many observers, with Herschel, thought it a living vol- 
cano. It is now, however, conceded by its best students 
that the Moon has no living volcanoes, and that the 
degrees in brightness can be accounted for by the nature 
of the materials reflecting the sunlight. 

From our acquaintance with the earth we can glean 
no idea of fhe torn nature of the Moon's surface, nor the 
extent of its territory eaten up by craters. Many attri- 



The Mo ox. 75 



bute the ragged appearance of our satellite to the small 
force of gravity upon its surface. Gravity upon the 
Moon's surface is less than one-fifth of what it is upon 
the earth's surface. The Moon draws down bodies and 
holds its own matter together with less than one-fifth of 
the earth's attractive force. For this reason it is argued 
that the internal forces, being less controlled, acted the 
more readily in upheaving the Moon's surface. 

Others say that the condition of the Moon's surface 
is due to the absence of water. The volcanic action of 
the earth has been much disguised by the effects of 
water, its sedimentary deposits covering up the work 
of fire. The Moon being void of water, the work of its 
internal fires is plainly visible. 

The Moon without Water. — If water existed on the 
Moon's surface it would rise in vapor, and the telescope 
could detect a cloud 200 yards in diameter. No cloud 
has ever been seen floating above the Moon. 

Again, a great body of water would reflect the sun's 
image in such a way as to be perceived by the telescope. 
Such a phenomenon has never been witnessed. 

The polariscope, moreover, adds its voice to the de- 
monstration of the non-existence of water on our satellite. 

The principle of the polariscope is, that if a ray of 
light is reflected or transmitted at certain angles of 
incidence, it is rendered incapable of being again 
reflected or transmitted excepting at certain angles 
of incidence. Two mirrors are so arranged in a frame 
that they may be placed at any angle to each other ; one 
of them is fitted to revolve, and has an index to note 
the degrees, and thereby determine its position in refer- 
ence to the other mirror. This delicate test shows that 
the sunbeams reflected from the Moon have touched no 
liquid surface. 



76 Astrokomt: New axd Old. 

The Moon has no Atmosphere. — The proof of this is the 
sudden extinction of the light of a star when occulted 
by the Moon. Twilight before the rising and after the 
setting of the sun is caused by the earth's atmosphere 
reflecting and refracting the rays of light. Had the 
Moon an atmosphere, however rare, the light of the star 
would be visible some time after the beginning, and 
before the end, of the calculated time of an occultation. 
This test would detect a lunar atmosphere with a 
density less than the one two-thousandth part of that 
of the earth's. 

Luminous Bands. — Great numbers of luminous bands 
appear, running along the Moon's surface from the tops 
of some of the high craters. More than a hundred of 
them run down the slopes of Tycho. Some observers 
consider them to be streams of volcanic matter erupted 
from the craters, supposing this lava to be a better 
reflector of light than the rest of the Moon's surface. 
Others think them fissures produced by the reaction 
of the Moon's interior on its exterior. 

A similar appearance results from filling a globe of 
glass with cold water, and pluuging it into warm water; 
the water confined in the glass expands, breaking the 
glass and producing radiating fissures. 

Rills — The rills differ from the luminous bands in 
being formed of two parallel slopes, with a sunken way 
between them. They are bright in the full Moon, but 
in the other phases dark, one of the ridges throwing its 
shadow into the trench. Formerly they were thought 
to be ancient river-beds ; but this is now given up, as 
they are known to cross high mountains and the sides 
of craters. The depth of the rills varies from 450 to 700 
yards. Their width often reaches a mile and a half, and 
their length varies from 10 to 125 miles. 



The Mo ok. 77 



Moon's Axes. — The Moon's figure is an ellipsoid with 
three unequal axes. The shortest axis is the one on 
which it revolves, and is almost perpendicular to the 
ecliptic or earth's path. The mean is that which lies in 
the direction of the Moon's motion of translation. The 
longest is the one that points towards the earth. It is 
supposed that the shortening of the axis of revolution 
is due to the action of centrifugal force while the Moon 
was in the plastic state. The lengthening of the long 
axis is attributed to the earth's action of gravitation 
upon the Moon when in the liquid or plastic condition. 
The effect of this attraction would be to draw the matter 
on this side of the Moon towards the earth, and draw 
the Moon away from the matter on the other side. The 
result of these forces would be to lengthen the Moon in 
the earth's direction. 

Librations. — The path of the Moon around the earth 
is elliptical or oval, and, as a consequence, it moves 
more rapidly in the parts of its path nearest to the earth 
than in the parts more remote. Its motion on its axis 
is uniform. Because the motion on the axis is uniform, 
and that in the orbit variable, the Moon reveals to us a 
little sometimes of one side and sometimes of the other 
that would otherwise be invisible. This is called the 
Moon's libration, or swaying in longitude. 

The axis of the Moon not being quite perpendicular 
to the plane of its path, our satellite, in one part of its 
revolution, reveals more of the region around one of the 
poles, and in another part, more of the region around 
the other pole, and so gives its axis the appearance 
of a tilting motion. This is called the Moon's libration 
in latitude. 

The moon has also a diurnal libration, caused by the 
earth's rotation. The Moon always turns the same face 



Astronomy: New and Old. 



to tlie earth's centre. We see it from the surface. 
When the Moon is on the meridian we view it as we 
would from the earth's centre ; but when the Moon is 
near the horizon our circle of vision takes in more 
of the upper limb than would be presented to a spec- 
tator at the centre of the earth. Hence we see a portion 
of one limb while the Moon is rising, which is gradually 
lost sight of, and we see a portion of the opposite limb 
as the Moon sinks to the west. 

Owing to these ligations, we see more than one-half 
of the Moon's surface. If the lunar surface be repre- 
sented ns 1,000, the part entirely invisible will be repre- 
sented by 424, and the part visible by 57G. The best 
students of the Moon say that there is no essential dif- 
ference between the portion of the Moon always visible 
to us and the seventh part, generally hidden from our 
gaze. From this is Concluded the similarity between 
the visible and invisible portions of the Moon. 

Eclipse of the Moon— An eclipse of the Moon is 
caused by the interposition of the earth between the 
sun and Moon. If the Moon's path corresponded with 
the ecliptic we would have a lunar eclipse every full 
Moon. As these paths are inclined to one another by 
a small angle, similar eclipses occur only after intervals 
of 18 years and 11 days, or 223 lunations. 

A total eclipse of the Moon may last two hours. 
If we add the partial eclipse on each side of totality, 
a lunar eclipse may continue four hours. 

The disc of the earth as seen from the Moon is 13.J 
times as large as the Moon's disc appears to us. When 
the earth's disc passes directly before the sun its 
shadow is more than great enough to cover the whole 
Moon at once, and there is a total eclipse present to 
every part of the Moon's surface at the same time. 



The Mo ok. 79 



An eclipse of the sun seen from the Moon is never so 
entirely obscure as an eclipse of the sun seen from the 
earth. This is due to our atmosphere. The solar rays, 
in traversing the lower strata of our air, are diverted, 
and purple the Moon with the tints of sunset. 

Moon's Heat. — Lord Eosse, by employing a large re- 
flector to concentrate the rays of the Moon at a focus, 
and using the delicate thermometer of the thermo- 
electric pile, found that the heat of the Moon produced 
on the earth an increase of one-fiftieth of a degree 
Fahrenheit. 

Lord Eosse estimated the change of temperature on 
the surface of the Moon, according as it was turned to 
or from the sun, at more than 500 degrees Fahrenheit. 
During the lunar night he computes the temperature at 
two or three hundred degrees below zero, and during 
the lunar day at as much above. The reason for this 
change is that the Moon, revolving on its axis during a 
lunation of 29^ days, has about fifteen of our days of 
day and fifteen days of night. During fifteen days the 
sun is incessantly pouring his heat upon the Moon, with- 
out modification from either atmosphere or moisture 
or clouds. During fifteen days of night the heat 
radiates into space, neither atmosphere nor cloud to 
interpose. 

Eecent experiments, however, would go to show that 
the temperature of the lunar surface is exceedingly low, 
even during its day-time, owing to the absence of atmos- 
phere. It is well known that the higher we ascend from 
the earth's surface the thinner grows the air and the 
colder it becomes. If we could go high enough to find 
the air entirely gone, the cold would grow intense, and 
while the direct solar rays would actually burn our skin, 
the thermometer would show the cold around us to be 



80 Astronomy: New axd Old. 

arctic. According to Ericsson, this applies to the Moon 
equally as to the earth, and on our satellite's surface, ow- 
ing to the absence of atmosphere, the mercury in the 
bulb of the thermometer would freeze never to melt 
again. Direct measures of lunar heat lately made con- 
firm this opinion, and indicate that the Moon's surface, 
even in sunshine, must be always cold. Ice once 
formed there would probably never melt. 

Is the Moon Peopled? — The Moon being void of air 
and water, and having so extraordinary a geology and 
climate, it is impossible to conceive it peopled by organic 
beings. 

The Moon as a Time-Piece. — The Moon is of great 
importance as a time-indicator. As the sun measures 
the year, so does the Moon the month. If we regard 
the sun as the hour-hand, the Moon is the minute-hand 
of the celestial dial. 

The Moon and the Mariner. — Latitude and longitude 
are the elements required by the seaman to compute his 
course. He can find his latitude from the sun or a star, 
but only the Moon can give him his longitude. 

The Moon and the Weather. — The Moon's influence on 
the weather, the clouds, and the atmosphere is imper- 
ceptible. 

Harvest Moon. — About the time of the autumnal equi- 
nox the Moon, when near the full, rises about sunset 
a number of nights in succession. This occasions a 
number of bright moonlit evenings at the harvest-time, 
lengthening, in a manner, the day, and enabling the 
farmer to gather his harvest. The Moon's path at dif- 
ferent seasons is differently inclined to the horizon. The 
inclination is least when the equinoxes are in the hori- 
zon, and the 13° which the moon travels eastward in 
her daily path will then carry her but a little way below 



The Mo ox. 81 



the horizon, and she will rise near the same hour for 
several evenings together. 

The Moon's services to the earth are manifold. She 
illumines the night ; measures time ; determines Easter 
Day, and consequently all the movable feasts of the 




Pig. 10.— Photograph of a Lunar Phase, by Prof. C. M. Charroppin, S.J., 
St. Louis University. 

I 

calendar; governs the tides, keeping the great bodies 
of water pnre and enabling seamen to launch their 
vessels ; and, as a celestial index, points ont to the 
mariner his path on the ocean. 



CHAPTER VI. 

THE TELESCOPE. 

Before approaching the planets it may be well to 
premise a short description of that marvellous instru- 
ment, the Telescope, which is a most important aid to 
their proper study. 

Galileo is credited with the invention of the Tele- 
scope. 

There are two kinds of Telescopes, refractors and 
reflectors. 

The Refractor. — A lens or prism has the property 
of refracting or bending a ray of light from its course, 
it being a principle of optics that light passing from 
a rare to a more dense medium is bent towards a per- 
pendicular to the refracting surface. The refractor, 
or refracting Telescope, has for an object-glass a lens 
(sounded by two convex surfaces, concave towards each 
other. This lens bends the parallel rays as they enter 
from their parallelism, and converges them to a focal 
point behind it. In this way the image of an object 
is formed at the focus behind the lens. Besides the 
large lens used as the object-glass, or objective, there 
is in the refracting Telescope an eye-piece or eye-lens, 
being a small lens to magnify and view the image 
formed at the focus of the large lens. These two, the 
objective and the eye-piece, are the essentials of a 
refractor. 

The Reflector. — Eeflectors, or reflecting Telescopes, 
form the image in the focus by reflecting the rays of 
light coming from the object by means of a concave 



The Telescope. 83 

parabolic mirror, made of polished metal or silver on 
glass. This mirror is called the speculum, and con- 
verges the parallel rays entering it to a focus in front, 
where the image is formed. This image must be viewed 
with an eye-piece similar to that of the refractor. 

Magnifying Powers.— The magnifying power of the 
Telescope is the quotient found by dividing the focal 
length of the objective by the focal length of the eye- 
piece. 

The rule of the French opticians for finding the 
highest magnifying power, usefully applicable to a 
Telescope, is to double the number of millimetres con- 
tained in the width of the aperture. A millimetre is 
the thousandth part of a metre, or the .03937 of an 
inch. An inch is equal to 25.4 millimetres. 

Chromatic Aberration. — The first Telescopes were 
refractors. A fatal property of the glass lens rendered 
useless the making of an object-glass larger than 
three inches in diameter, and it required a century and 
a half to obviate the difiiculty. Chromatic aberration 
was the obstacle. When light passes through a prism 
or lens two distinct operations take place, refraction 
and dispersion. Refraction is the bending of the rays 
from their course when passing from a rare to a more 
dense medium, and it is this property of a lens that 
gives it its utility as a telescope. 

The power a lens has of separating the colored rays 
in a beam of light is called dispersion. Yiolet is dis- 
persed to the greatest distance from the primitive 
course, and red to the least, and the other colors inter- 
mediately. This is chromatic (color) aberration (wan- 
dering-away), or the separation of the component colors 
of light. 

Achromatism. — Euler, the great German mathemati- 



84 Astronomy: New and Old. 

cian, was the first to remedy chromatic aberration in 
theory, and Dolland, an English optician, in practice. 
This was accomplished by combining into one objective 
two lenses having the same dispersive, and unequal 
refracting, power. Crown and flint glasses were the 
materials used. The dispersion of flint-glass is double 
that of crown-glass, while their refraction is almost 
equal. By combining a convex lens of crown with a con- 
cave lens of flint of about half the crown's curvature we 
will have refraction without dispersion. The crown- 
glass is on the exterior. A lens so composed is said 
to be achromatic, or colorless. In the best object-glasses 
now used the crown is a double convex lens, and the 
flint nearly a plano-concave. In this way the two 
Lenses tit exactly together, the concave of the flint 
fitting the convex of the crown, while the inner face 
of the Hint is nearly flat. No two specimens of glass 
being precisely similar with respect to their relative dis- 
persion and retraction, the optician must experimentally 
find (heir ratio in every objective he makes. The lenses 
arc usually joined witli transparent balsam or castor- 
oil. 

Secondary Aberration.— In very large achromatic objec- 
tives a secondary aberration arises from the fact that 
flint-glass, as compared with crown-glass, disperses 
the violet end of the spectrum more than the red, and, 
consequently, throws a violet or blue areola around the 
object. No art can correct this defect, as it flows from 
the properties of the glass itself. It may, then, be said 
that we have attained the limit of power in the refrac- 
tor. A refractor of thirty-six inches in diameter in all 
likelihood touches the acme of perfection. 

The Eye-Piece.— The eye-piece of a Telescope is, like 
the objective, formed of two glasses. An additional 



The Telescope. 85 



lens, called the field-lens, is added to the eye-piece in 
most Telescopes for the purpose of gathering up the 
outer rays from the objective, and so aiding the eye-lens 
in giving distinctness to the image. The eye-piece is 
said to be positive when the field-lens is located between 
the eye and the image, and negative when the field-lens 
is so placed that the light beams are passed through 
it before reaching a focus. 

The short focal distance of the opera- glass is due to 
its having the negative eye-piece. 

Manner of Using.— The two principal ways of using 
the Telescope are, as an equatorial and a transit instru- 
ment. 

The transit instrument is fixed immovably in the 
plane of the meridian, and directed towards the south. 
It catches but a momentary glimpse of the celestial 
object as it is rapidly carried into the field of view by 
the earth's diurnal revolution, and its use is to deter- 
mine the motion of the celestial bodies. It is the 
mathematical instrument of astronomers. 

The equatorial is so arranged that it follows the daily 
revolution of the celestial sphere, and is thus enabled to 
keep the same object constantly in the field of view for 
the purpose of thorough investigation. Usually, in the 
case of large equatorials, there is a clock-work attach- 
ment to furnish the necessary motion. The equatorial 
is the weapon of the descriptive astronomer. 

Among the greatest refractors in existence is the 
26-inch Washington equatorial, completed in 1873, and 
remarkable for its discovery of the two satellites of 
Mars, the most diminutive planetary bodies known to 
Astronomy. It held the primacy among refracting Tele- 
scopes for nearly eight years, when it was succeeded, in 
1880, by the 27-inch refractor, made by Howard Grubb, 



86 Astronomy: New and Old. 

of Dublin, for the Vienna Observatory. This was, in 
turn, succeeded by the 30-inch retractor made by Alvan 
Clark for the Pulkowa Observatory. Lastly conies the 
great 3G-inch equatorial made by Alvan Clark for the 
Lick Observatory. It was first directed to the heavens 
from the summit of Mount Hamilton on the evening 
of January 3, 1888. It is a great success, and has 
already accomplished some wonderful feats. Among 
other things, it has given clear views of the tiny satel- 
lites of Mars when, owing to the planet's great distance, 
they were six times as faint as when discovered by Pro- 
fessor Hall with the Washington refractor. 

Reflecting Telescopes — The reflecting Telescope did 
not come into use for more than a century after the 
retractor, although its principle was suggested by Mer- 
senne in 1639. AVith the reflector the image is formed 
in front of the mirror or speculum, and hence a difficulty 
arises in endeavoring to remove the observer out of the 
direct path of the rays from the object. 

There are three plans for avoiding this difficulty, 
devised respectively by Gregory, Newton, and Herschel. 

Gregorian Telescope — James Gregory, who is usually 
credited with making the first reflector, placed a small 
concave mirror behind the focus, by which the light is re- 
tleeted back again to a small opening in the centre of the 
speculum, where the image is formed, and may be viewed 
through an eye-piece fixed into this central opening. 

The Cassegrainian Telescope — The Cassegrainian 
Telescope is the same in principle with the Gregorian, 
and differs only in having the small mirror convex 
instead of concave, and so placed between the speculum 
and its focal point that the rays do not really come 
to a focus until they reach, by a second reflection, the 
eye-piece in the speculum's centre. 



The Telescope. 87 



The Newtonian. — Newton placed a small plane mirror 
just inside the speculum's focus, making an angle with 
the axis of the Telescope of 45°, and so throwing the 
rays to the side of the tube, where they are focussed 
and form the image. An opening is made in the tube's 
side opposite the small mirror, into which an eye-piece 
is screwed. 

The Herschelian. — Herschel's plan was to slightly tip 
the speculum so that the rays were focussed and the 
image formed at the upper margin of the tube, where 
an eye-piece was fastened. The observer, in viewing 
an object with the Herschelian telescope, must take his 
position at the upper end of the tube and look directly 
into it, turning his back to the object. 

It is on the Herschelian principle, though with an 
additional mirror, that the mightiest reflector in exist- 
ence is made. This is the telescope built by Lord Bosse 
at Parsonstown, in Ireland. Its speculum is six feet in 
diameter, and is made of an intractable alloy of two 
parts of copper to one of tin, admitting of a very bright 
polish. The Telescope is sixty feet long, and is the 
most powerful in the world. 

In viewing a star the Telescope does not increase 
its size, for the star, owing to its awful distance, is but 
a point, and how often soever we may multiply a point 
it still remains a point, but it increases its brilliancy 
by gathering up more of its light rays. The diameter 
of the pupil of the eye is about one-fifth of an inch, and 
as surfaces are to each other as the square of their like 
dimensions, the amount of light received from a star 
through the Telescope will be as one fifth squared to the 
square of the diameter of the aperture. An objective 
of six inches gives an increase of nine hundred times. 
Hence the brilliancy of a star as seen through Tele- 



88 Astronomy: New and Old. 



scopes will be proportional to the square of the 
diameter of their apertures. When viewing a planet 
the Telescope increases its apparent size in the ratio 
of the focal distance of the objective to that of the eye- 
piece. But the amount of light is not increased, as in 
the case of a star. The reason is because the light has 
to be diffused over a larger surface— larger in propor- 
tion to the magnifying power. 

There is a dispute concerning the relative merits 
of reflectors and retractors. As a summary of the dis- 
cussion, it may he said that the retractor is the more 
durable and better working instrument, and the reflector 
the more powerful. 

Limit of Power.— Is there a limit to the magnifying 
power of tl.e Telescope I There is ; and it is owing, not 
to the Telescope, but to the atmosphere. 

The air is always in a state of waviness or trembling, 
occasioned by the cool air descending and the heated 
air ascending. The greater the magnifying power 
employed the more is this disturbance perceptible, so 
that when the power is beyond 1,000 a star, instead 
of appearing as a point, looks like a stream or glare 
Of light. Climate may, in a measure, modify this 
difficulty ; but it is safe to say that a magnifying power 
much beyond 1,000 diameters cannot be usefully 
applied. 

The Micrometer.— The micrometer consists of a tube, 
across the opening of which are stretched two parallel 
threads. These threads are so arranged that they can : 
be moved to or from each other by the turning of 
a screw. 

The paraUel threads are crossed by a third one per- 
pendicularly. 

This little contrivance is placed in the focus of a 



The Telescope. 89 



Telescope, and the distance apart of two stars may be 
measured with great accuracy by adjusting the parallel 
threads respectively to the centres of the stars, and 
counting the number of turns and fraction of a turn 
required to bring the threads together. A graduated 
scale is devised for the turning-screw. 

The micrometer is used in measuring the apparent 
diameters of the heavenly bodies, and other minute 
celestial distances. 

The Heliometer. — This is an instrument devised for 
the accurate measurement of the apparent diameter of 
the sun or heavenly bodies. It is a Telescope the 
object-glass of which is cut vertically in half. One half 
is fixed in the tube, and the other half is movable, being 
mounted on a slide. Each half of the object-glass will 
form in the eye-piece of the Telescope a perfect image 
of the sun or heavenly body. A screw of delicate 
mechanism, with a graduated scale, moves the slide. 

If the sun's diameter is to be measured, the halves 
of the lens are adjusted so that the images may touch 
one another, and then the distance between their centres 
will give the diameter of the sun in seconds. 

The Meridian Circle. — The meridian circle is a large 
graduated circle attached to the axis of the transit 
instrument. The number of degrees through which the 
transit Telescope turns is easily read off from this 
circle. It is chiefly used to find the right ascensions 
and zenith distances of heavenly bodies. 



CHAPTER VII. 

THE PLANETS. 

Among the host of stars visible to the unaided eye 
there are five that are seen to continually change their 
place and wander about among the others. These 
live were known to the ancients. They move about 
capriciously in the starry ranks, and for this reason 
were called by them Planets, or wanderers. 

The Planets known to antiquity are Mercury, Venus, 
Mais, Jupiter, and Saturn. 

The telescope has added to the number of the planet- 
ary bodies two other great ones, Uranus and Neptune, 
and two hundred and eighty-one lesser ones. 

The Planets are the associates of the earth in space, 
and with it belong to the family of the sun. 

All the Planets alike borrow their light from the sun. 

When viewed with the telescope, the fixed stars 
remain as points of light, while the Planets present 
a visible disc. 

The unaided eye distinguishes the Planets by their 
steady radiance. The twinkling of the fixed stars is 
probably due to the interference of light in its awful 
journey through space. 

The distinguishing characteristic of a Planet, how- 
ever, is its motion among the stars. 

The planetary bodies are divided into two classes, 
the Major and Minor Planets. The eight large Planets, 
of which the earth is a member, are called the Major 
Planets, from their size and importance, and the lesser 
ones the Minor Planets. 

90 



The Planets. 91 



Tlie Major Planets are sub-divided into two groups, 
the inner and the outer group. The inner group em- 
braces Mercury, Venus, the Earth, and Mars. The 
outer embraces Jupiter, Saturn, Uranus, and Neptune. 

All the Planets revolve about the sun, are members 
of his family, and are governed and held in place by 
his great mass. 

The orbits of the Planets are all similarly shaped, 
being ovals. All the Planets revolve in nearly the 
same plane, that of the Ecliptic. 

There is still another division of the great Planets 
into inferior and superior. The inferior Planets have 
their orbits within the earth's, and are Mercury and 
Venus. The superior Planets have orbits beyond the 
earth's, and are Mars, Jupiter, Saturn, Uranus, and 
Neptune. 

The six outermost Planets are attended by 20 satel- 
lites. The Earth has 1, Mars 2, Jupiter 4, Saturn 8, 
Uranus 4, and Neptune 1. 

The direction of the orbital motions of the Planets 
is from west to east, or against the hands of a clock. 
This is, too, the direction of their rotations. It is also 
the direction of the motion of the satellites arouud their 
primaries, except in the case of those of Uranus and 
Neptune. 

It is surmised that there is a law governing the 
periods of the axial rotation of the Planets, but it has 
not been established. 

It is suspected that two undiscovered Planets exist 
beyond the orbit of Neptune. Two families of comets 
are known to travel in that distant region, and their 
movements would seem to indicate the presence there 
of two large planetary bodies. It is a mere sus- 
picion. 



92 Astronomy: New and Old. 

ZtfERCURY. 

Mercury is the nearest Planet to the sun, and it was 
known to the ancients. Neither the name, nation, nor 
epoch, however, of its discoverer has been preserved. 
The first authentic record we have of an observation 
of the Planet is of one made in the year 265 B.C. 

Mercury has phases like the moon, and they are 
explained on precisely the same principles as the lunar 
phases. 

Mercury is an evening and a morning star. Mer- 
cury is v«iy difficult of observation, being so near to 
tin- sun that it is usually swallowed up in his rays. It 
shines with a bright white light, and is, perhaps, equal 
in lustre to Sirius. 

The apparent size of its disc, as seen through the 
telescope, varies from 5" to 12", according as it is viewed 
at its nearest or farthest approach to us. When beyond 
the sun Mercury appears round and small ; and when 
nearly between the earth and sun it takes the form 
of a slender crescent. It is supposed to rotate on its 
axis in about 21 hours. The exact period of its rota- 
tic ui is not definitely known. 

The Planet makes a revolution around the sun in 
nearly 88 days. Its average distance from the sun 
is 30,000,000 miles. It travels in its orbit with an 
average velocity of over 29 miles a second, or a hundred 
times more rapidly than a rifle bullet. 

Its oval path departs from the circular form much 
more than that of any other of the Major Planets. Its 
greatest distance from the sun is 43,000,000, and least 
30,000,000, miles. As a consequence, its velocity varies 
in the different parts of its orbit from 23 to 35 miles 
a second. 



The Plaxets. 93 



When Mercury is farthest from the sun the solar 
heat beats down on its surface with more than four 
times the intensity that it ever attains at the earth's 
surface ; and when the Planet is closest to the sun this 
heat is nine times as great as the most intense that ever 
touches the earth. 

The seasons on Mercury, owing to its short year, 
change much more rapidly than they do upon the earth. 
It is only 44 days on Mercury from mid- winter to mid- 
summer. In the space of six weeks the sun rises to 
Mercury to more than double his apparent size, and 
radiates upon the Planet more than double the quantity 
of light and heat. 

A luminous margin has been seen by observers on 
different occasions during Mercury's transit across the 
sun's disc, and variously estimated at from one-third to 
two-thirds of the Planet's diameter in depth. From 
this it is thought that the Planet has a dense atmos- 
phere, The measurements of the intensity of the Planet's 
light, however, render doubtful the very existence of an 
atmosphere. The climate of Mercury, even if it has 
a dense atmosphere, must be an extraordinary one. 

Von Asten gives the weight of Mercury as the one 
twenty-fourth of the earth's. 

The elongation of Mercury, or its departure, never 
exceeds 29° on each side of the sun. The Planet is 
sometimes above the horizon for two hours after the 
disappearance of the sun. The most propitious times 
to see Mercury with the unassisted eye are the spring 
for morning, and the autumn for evening observation, 
as the Planet is at those times north of the sun, and 
almost vertically over the place of sunset and sunrise. 

Dark and bright markings, of a changeable nature, 
have been noticed on the Planet's disc, and many 



94 Astronomy: New and Old. 

recent observers have regarded them as similar in 
character to those oil Mars. 

Mercury's nodal returns recur, under nearly the 
same conditions, after periods of thirteen years. The 
transits of this Planet across the sun's disc happen 
after average intervals of less than ten years. The 
longest interval between two successive transits is 
thirteen years. 

The first recorded transit was observed by the 
illustrious Gassendi, on November 7, 1G31. These 
transits always occur in May or November, because 
the Planet crosses the ecliptic in these months. The 
next transits will be on May 9, 1891, and November 
10, 1894. 

Mercury rotates on an axis inclined 70° to the 
plane of its path. 

The spectroscope shows that the light of Mercury 
is reiiected sunlight, and its spectrum is a faint echo 
of the solar spectrum. The spectroscope neither con- 
firms nor denies the existence of a Mercurial atmos- 
phere. 

The theory of an intra-Mercurial Planet, hypothet- 
ically called Vulcan, is abandoned. 

VENUS. 

Venus has no rival in brilliancy among the starry 
host. The Chaldean shepherds praised its glowing 
beauty with such fervor that the East called it the 
shepherd's star. Hipparchus and Ptolemy called it 
Venus, probably on account of its matchless lustre 
and incomparable loveliness. Its brightness is, indeed, 
sometimes so intense that in a very clear sky it is 
visible by day. 



The Planets. 95 



Yenus is the second Planet in the order of distance 
from the sim. It is an inferior Planet, since its orbit 
lies between the earth and sun. 

The Planet shines by reflected sunlight, and it has 
phases like the moon. Owing to its distance, it shows 
no disc to the unaided eye, and its crescent form can- 
not be perceived without telescopic aid. 

Its mean distance from the sun is about 67,000,000 
miles. 

Its orbit is nearer to a perfect circle than any other 
of the planetary paths. 

Its day is 39 minutes shorter than ours. It revolves 
on its axis in about 23 hours and 21 minutes. There is, 
however, it must be confessed, considerable uncertainty 
in respect to this matter. 

Venus traverses its orbit in 224.7 days. Its speed 
is next to that of the winged Mercury, being on an aver- 
age 22 miles a second. 

The Planet at times approaches as near to the earth 
as 24,000,000 miles, and again, at times, it recedes to 
a distance of 162,000,000 miles. Owing to this great 
difference in distance, the Planet presents very dif- 
ferent aspects when viewed at different times through 
the telescope. 

Yenus never departs from the sun beyond 47°, and 
is thus visible but a short portion of the night. 

The Planet oscillates from one side of the sun to 
the other, and becomes alternately morning and evening 
star. As morning star it was known to the ancients 
as Phosphorus ; and as evening star, Lucifer. 

The equator of Yenus makes an angle of 53° 11' 26" 
with the plane of its path, so that its axis of rotation 
departs much more from a perpendicular to its orbital 
path than does the earth's. 



00 - 1 s 77,' o Arojf f : JVfa w a yj) Ol d. 



The diameter of Venus is 7,660 miles, or 258 less 
than that of the earth. The mass of the Planet is 
computed at three-quarters of that of the earth. The 
density of the Planet is about .85 of the earth's, and 
it would weigh 4.81 times as much as an equal-sized 
globe of water. 

The force of gravity on the Planet's surface is a 
little less than on that of the earth. Near the earth's 
surface a body abandoned to gravity would fall 16JU 
feet in the first second; it would fall three feet less 
near the surface of Venus. 

Astronomers take advantage of the transit of Venus 
to compute the distance to the sun. These transits 
are very rare. They occur in pairs, after intervals 
of 121} and 105J years. The pairs are separated from 
one another by eight years less l>\ days. They occur 
in pairs separated by eight years because eight revolu- 
tions of the earth around the sun are performed in 
nearly the same period as thirteen revolutions of Venus. 
Alter eightyears Venus and the earth are back in a 
line again, and near the same position relatively to 
the sun. 

A short time before the beginning and after the end 
of a transit the globe of Venus is faintly perceptible. 
This is due td the Planet's atmosphere. It is thought 
that the atmosphere of Venus is somewhat more dense 
than the Earth's. 

The spectrum of Venus does not materially differ 
from the solar spectrum. The spectroscope testifies, 
in a measure, to the similarity of the atmosphere of 
Venus and our own. 



The Plaxets. 97 



THE EARTH. 

The Earth is the third Planet we reach as we travel 
outward from the sun. Its path is elliptical, or oval. 
Its mean distance from the sun is 92,500,000 miles. It 
is 3,000,000 miles nearer to the sun on January first 
than on July first. 

There are many proofs of the Earth's rotundity. The 
first is from analogy. The sun, moon, and the planetary 
bodies are all round, and, accordingly, Ave conclude our 
own Planet to be similarly shaped. The Earth's sha- 
dow, cast on the moon during a lunar eclipse, is circular; 
the circumnavigation of the globe ; the horizon is seen 
to expand as we ascend a mountain ; and the polar star 
is perceived to sink or rise as we journey to or from the 
equator — these are other evidences of the Earth's 
sphericity. 

The figure of the Earth is usually referred to as an 
oblate spheroid, or ellipsoid of revolution. This is 
really the figure that a molten fluid mass of matter of 
the size of the Earth, and whirling on an axis with the 
Earth's axial rapidity, would assume. The centrifugal 
force of such a revolving body would produce a project- 
ing bulge at its equator and a contraction at its poles. 

Geologists contend that the Earth's interior is now 
an ocean of liquid fire, and that the hard exterior crust 
is but the merest shell. As evidences of the awful heat 
of the Earth's deep interior they point to boiling 
springs, volcanoes, earthquakes, and particularly to the 
supposed gradual increase in the temperature of all 
deep mines of 1° F. for every 55 feet of descent after the 
first 100 feet. 

The theory of regular increase is now, however, 
abandoned. 



98 Astronomy: New and Old. 

Admitting that there are really enormous quantities 
of heat in the Earth's interior, and such as, under ordi- 
nary circumstances, would reduce the metals to a liquid 
state, still the science of mathematics insists that the 
Earth is a rigid body, due, probably, to the intense 
downward pressure of its parts. The best mathematical 
analysis declares that the Earth must have the rigidity 
of steel that the phenomena of the tides may be satis- 
factorily explained. It is the difference in the behavior 
of the solid and Liquid portions of the Earth under the 
influence of solar and lunar gravity that produces the 
tides. If the Earth were a vast ocean of liquid fire, 
with a thin crust or covering, there would be no tides, 
tor the continents and seas w r ould rise and fall together 
under the action of an external attraction. Moreover, 
dreadful tides would continually rise in this mighty 
igneous ocean, and tlow around after the moon and sun. 
No crust could withstand the breaking of these tides. 

Cavendish, by means of a torsion balance, endeavored 
to calculate the mass of the Earth. It is a law of gravi- 
tation that bodies attract in proportion to their mass. 
He compared the Earth's attraction with that of an equal 
bulk of lead. 

Bailie and Cornu, by an improvement on the process 
of Cavendish, computed that the Earth weighs 5.55 as 
much as a globe of water of the same size. 

The height of the pole above the horizon is the lati- 
tude of an observer, or his distance from the equator. 
By means of the polar star a degree of the meridian can 
be measured. From careful measurements made on the 
Danube between Hammerfest and Ismailia, and in 
India, it is computed that the equatorial radius of the 
earth equals 6,377,377 metres, and the polar radius 
equals 6,355,270 metres, and the ellipticity ^rsi^s- 



The Planets. 09 



Pendulum measurements may be said to confirm 
these results. 

The Earth has strictly no purely geometrical figure, 
and the nearest approach to its exact form would be an 
ellipsoid of three unequal axes. 

MARS. 

Mars is the first Planet whose orbit lies outside that 
of the Earth, or it is the first superior Planet. Of all 
the Planets, it bears the closest resemblance to the earth. 
Its surface is less obscured by clouds and vapors than 
that of any other of the Major Planets, and, on account 
of this and of its favorable position, we are better 
acquainted with the details of its surface than with 
those of any other heavenly body except the moon. 

Its mean distance from the sun is about 141,000,000 
miles. Its least distance is 128,000,000, and its greatest 
154,000,000, miles. From this it is seen that its orbit 
departs considerably from the circular form. Indeed, 
the path of Mars has, next to that of Mercury, the 
greatest eccentricity of any of the orbits of the great 
Planets. 

Mars is nearest to us when it is in opposition, 
or on the same side of the sun with the earth. 
It then may approach to within 35,000,000 miles of 
the earth. 

Mars makes a circuit of its orbit in 687 days. Be- 
cause the year of Mars is not a multiple of our year, its 
oppositions will occur at irregular intervals. Very 
favorable oppositions, however, recur at intervals of 
fifteen years. The Planet is much nearer to us at some 
oppositions than at others, owing to the difference in 
form of the orbits of Mars and the earth. The earth's 



100 Astronomy: New axd Old. 

orbit is nearly circular, and that of Mars very eccentric. 
The most favorable oppositions will be those occur- 
ring closest to the 26th of August. 

Mars is more than four times as bright at the most 
favorable as it is at the most unfavorable oppositions. 
The last favorable opposition was in 1877, and the next 
two will occur in 1892 and 1909. 

To the unaided eye Mars usually appears as a star 
of the first magnitude, of a ruddy color; indeed, it is 
the reddest star in the sky. The cause of its color is 
unknown. 

The equator of this Planet is inclined to its path by 
an angle of 27°, so that the change of seasons on Mars 
is much more marked than on the earth. 

The diameter of the plauet is 4,200 miles. The 
volume ot Mars is one-seventh of the earth's, double 
that of Mercury, and seven times that of the moon. 
The mass of Mars is a little more than an eighth part 
of the earth's. The force of gravity, or the weight 
of bodies, on Mars is one-half of that on the earth. 

The atmosphere of Mars is much rarer than the 
earth's, and atmospheric pressure on the Planet's sur- 
face, instead of being fifteen terrestrial pounds to the 
square inch, as it is with us, is computed to be about two 
and a quarter pounds. 

The surface of the Planet is thought to be about 
equally divided between land and water. 

The polar snows are plainly visible upon Mars. 

The markings on the Planet's surface are permanent 
features, and cannot be attributed to the presence of 
floating clouds. The telescope shows dark gray or 
greenish patches upon a deep yellow ground. On 
account of the permanence of these markings, the time 
of rotation of the Planet on its axis has been deter- 



The Planets. 101 



mined with the greatest accuracy. The period of rota- 
tion is 24 hours, 37 minutes, 22.7 seconds, and this is the 
length of the Martian day. 

Schiaparelli, director of the Milan Observatory, 
claims to have discovered some curious features upon the 
Martian surface. What was formerly regarded as Mar- 
tian continents he declares to be numbers of islands, 
separated from each other by a network of dark lines 
which he calls canals. These canals appear to be exten- 
sions of the Martian " seas," to connect them together, 
share their gray-green color, run in straight lines three 
or four thousand miles, and to be of a nearly uniform 
breadth of about sixty miles. The appearance of these 
peculiar formations on the Martian surface has been 
confirmed by other observers. 

During the opposition of 1881-2 Schiaparelli an- 
nounced that he saw not only these canals, but also 
other similar canals running along parallel to the first. 
He has drawn a marvellous map of the Martian surface, 
spread all over with pairs of parallel canals. The latest 
views of the Milan observer have not been implicitly 
received, and await confirmation. 

Professor Asaph Hall, with the 26-inch refractor of 
the Washington Observatory, discovered, during the 
opposition of 1877 (August 11), two tiny satellites to the 
Planet Mars. The nearest to the Planet has been 
named Deiinos, is about 5,800 miles from the Planet's 
centre, and revolves round the primary in 7 hours, 
39 minutes, 14 seconds. The outer satellite is called 
Phobos, is distant 14,600 miles, and makes a circuit of 
the Planet in 30 hours, 17 minutes, 54 seconds. Deimos 
is about 18 miles in diameter, and Phobos 22J. 

While Mars turns once on its axis, the inner satellite 
has sped around it three times. This is a striking ano- 



102 Astroxomy: New and Old. 

maly in the planetary system, as it was always thought 
that the satellite could not revolve with greater speed 
than that with which the Planet rotates. The extraor- 
dinary feature of our system is supposed to be due 
to a retardation of the Planet's axial motion by the 
solar tides. 

The path of Mars is inclined 1° 51' to that of the 
earth, and the planet travels in its orbit at a speed 
beyond fifteen miles a second. 




Fig. 11.— Views of Mars at two hours' interval. (Warren De La Rue.) 
THE MINOR PLANETS. 

The Minor Planets, or Planetoids, all revolve in the 
space between Mars and Jupiter. In the matter of size 
these small Planets are insignificant, ranging from 12 to 
500 miles in diameter, and are entirely invisible to the 
unaided eye. 

Vesta is the largest of the group, and Professor Pick- 
ering, from determinations of light-intensity, computes 
its diameter at 319 miles; while Professor Harrington, of 
Ann Arbor, assigns it a diameter of 520 miles. Picker- 



The Planets. 103 

ing places the diameter of Pallas at 167, that of Juno at 
1)1 miles ; and Harrington holds that the size of Yesta 
and Flora together nearly equals that of all the rest. 

Leverrier calculated that the mass of all the plane- 
toids together could not possibly exceed one-fourth of 
the earth's mass ; but it is very probable that this esti- 
mate is much too high, and that their united masses can- 
not exceed the s J ff of the mass of the earth, or ¥ V of 
that of Mercury. 

The number of the Planetoids now known is 281. 

They travel around the sun, similarly to the great 
Planets, from west to east in elliptical orbits. 

The mean eccentricity of their orbits is less than 
that of Mercury, and the average plane of their paths 
differs but slightly from that of the sun's equator. 

The path of Pallas has the greatest inclination 
of any of the asteroidal orbits, and its angle is 34° 42'. 

It is almost certain that these small Planets have no 
atmospheres. 

Astronomers since Kepler's time have considered the 
void between Mars and Jupiter as extraordinarily great, 
and suspected the presence there of an undiscovered 
planetary tenant. An alleged law, discovered by Bode, 
strengthened this suspicion. 

Bode took this series, 0, 3, 6, 12, 24, 48, 96, and add- 
ing 4 to each, formed the series, 4, 7, 10, 16, 28, 52, 100. 
Excepting the fifth number, 28, these figures almost rep- 
resent the proportion of the distances of the Planets 
from the sun. The following are really the relative 
distances of the Planets from the central orb : Mercury, 
3.9; Venus, 7.2; Earth, 10 ; Mars, 15.2; Jupiter, 52.9 ; 
Saturn, 95.4. 

Although we now know that this discovery of Bode 
is but a coincidence and no law, still its announcement 



104 Astronomy: New and Old. 

led observers to greater industry in their search for 
planetary bodies. 

Piazzi, of Palermo, discovered the first asteroid on 
the first of January, 1801, and called it Ceres. In prose- 
cuting his search, Piazzi resolved to examine all the 
stars in that part of the heavens bordering* on the eclip- 
tic. He examined a group of fifty stars at a time. He 
examined each group four times in succession before 
passing to a new group. The thirteenth star in the one 
hundred and fifty-ninth group was found to be a small 
Planet. It was discovered in the constellation Taurus, 
and appeared to be a star of the eighth magnitude. 
After a few observations, Ceres passed into a portion 
<»f its orbit where it was lost to sight in the rays of the 
sun. 

Gauss, however, the great German mathematician, 
had discovered a method by means of which he could 
compute a Planet's orbit from three observations. 
When Ceres emerged again from the sun's rays Gauss 
had calculated that its path lay between the orbits 
of Mars and Jupiter, and was able to point out its place 
among the stars, so that it was immediately seen by 
observers. 

In March, 1802, Olbers found a second planetoid, 
revolving also in the space between Mars and Jupiter, 
which he called Pallas. 

Harding, of Lilienthal, caught a third small Planet 
in 1801, and called it Juno ; and Olbers found another 
in 1807, and named it Vesta. 

Thirty -eight years after the finding of Vesta, Hencke, 
of Driessen, discovered the fifth planetoid. After this 
discoveries followed rapidly, until we now have 281 of 
these lilliputian neighbors. 

Palisa, of Vienna, discovered 60 planetoids j Peters, of 



The Plaxets. 105 



Clinton, IT. S. A., 47 ; Luther, of Bilk, 23 ; and Watson, 
of Ann Arbor, U. S. A., 22. 

The elements of the orbits of the planetoids, and 
other records concerning them, are published in the 
Berlin Year-Book. 

Olbers, on discovering the great inclination of the 
orbit of Pallas, suggested his famous hypothesis, that 
the planetoids are the fragments of a disrupted great 
Planet that once filled the void between Mars and 
Jupiter. This hypothesis of Olbers is now abandoned. 

Astronomers hope that in the near future the dis- 
tance to the sun will be able to be computed with great 
accuracy through means of the minor Planets. The 
efforts in the past to find this distance correctly through 
the agency of Venus and Mars have failed of success, 
owing principally to the refraction of their atmospheres, 
and the rarity offered by them of an opportune position. 

The small Planets have no atmospheres, and some of 
them approach so near the earth that their distances 
from us can be measured. About a dozen of them are 
little further away from us than three-quarters of our 
distance from the sun, and one or two will be suitably 
placed every year for this measurement. 

Kepler's third law governs the relative distances of 
all the Planets from the sun. If, then, we can find the 
distance of one Planet from us, we can find our distance 
from the sun. 

The planetoids are unequally distributed over the 
zone they occupy, but the greatest number are grouped 
at a mean distance from the sun 2.8 times that of the 
earth's. 

Professor Daniel Kirkwood, of Indiana University, 
maintains that Jupiter strongly influences the plane- 
toids, and has helped considerably to shape their orbits. 



100 Astroxomy: New axd Old. 



Jupiter is the largest of the planetary bodies, and his 
orbit is the first beyond those of the swarm of plane- 
toids. 

Mass. — The mass of Jupiter is greater than the united 
masses of all the other Planets, great and small. 

Distance. — Jupiter revolves at a mean distance from 
the sun of 482,000,000 miles, and the diameter of his 
orbit is 5.2 times that of the earth's. 

Shape of Path. — The path of Jupiter around the sun is 
decidedly elliptical, the Planet's greatest distance from 
the central orb being 5.45, and least 4.95, times the 
earth's mean distance. 

Speed. — The greater is a Planet's distance from the 
sun, the slower is its orbital motion; and Jupiter's 
velocity* of only 8 miles a second to the earth's 18 is an 
exemplification of this. Jupiter makes a circuit of its 
great orbit in 4,332.G days. 

Time of Rotation. — Jupiter rotates on its axis in about 
hours and 55 } 2 minutes. His axial motion is much 
more rapid than the earth's. 

Diameter. — Jupiter's mean diameter is about 85,000 
miles, his equatorial being 87,500 and polar 82,500 miles. 
His equatorial bulge is much greater than the earth's, 
due probably to the greater centrifugal force of his much 
higher axial speed while in the plastic state. A point 
on Jupiter's equator rotates 27 times more rapidly than 
one on the earth's. 

Not Solid. — It is concluded, for many reasons, that 
Jupiter is not a solid body. Jupiter's mass has been 
computed from the amount of gravitational influence he 
exercises on his satellites or moons. This computation 
has been confirmed by his action in shaping the paths 



The Plaxets. 107 



of the minor Planets, and also in influencing the orbits 
of comets. Jupiter's mass is about 310 times that of the 
earth's, and his volume is computed as more than 1,200 
times the earth's volume. From these figures it is easily 
seen that the specific gravity of Jupiter is not only less 
than that of the earth, but very little more than that of 
water. Indeed, the Planet's density is only 1.38 times 
that of water. 

Primitive Heat. — It is more than probable that Jupiter 
is at present in a highly heated condition, having still 
retained a large amount of its original caloric. The 
changing features of the Planet's disc are indices of 
its internal agitation, caused by great stores of heat. 

The Markings not Permanent. — The belts, or markings, 
on Jupiter are continually undergoing changes. Some 
of the changes are caused by the Planet's rotary motion, 
which in the space of five hours carries one whole hemi- 
sphere away from view, and replaces it by the opposite 
one. But, apart from the rotary changes, there are 
others, as new belts and fresh features are constantly 
appearing. Indeed, there are no permanent markings 
on the Planet. 

Violent storms are known to sweep over the Planet's 
disc which cannot be occasioned by the sun's heat, since 
at Jupiter's surface its intensity is only the ^ of what 
it is at the earth's surface. These storms are evidently 
caused by internal fires. 

Not Self-Luminous. — This internal heat is not, how- 
ever, sufficient to make the Planet self-luminous. Ju- 
piter's brightness is reflected sunlight. The proof that 
Jupiter is not self-luminous is that the satellites throw 
shadows upon the primary when they pass between it 
and the sun, and the Planet renders the satellites 
invisible when it passes between them and the sun. 



108 Astronomy: New axd Old. 

Jupiter reflects 62 per cent, of the light-rays imping- 
ing upon it. It is thought, from this great brilliancy, 
that the Planet must have some original luminosity. 

Hoggins and Vogel give the spectrum of Jupiter as 
showing the Fraunhofer lines belonging to reflected 
sunlight, and some lines due to atmospheric absorption. 
Some of these latter belong to aqueous vapor, some are 
unknown, and one agrees with a line in the spectra 
of some red stars. There is also observed in the spec- 
trum of the Planet an absorption of blue rays, due to 
great depth of atmosphere. None of these lines would 
point to intrinsic light. 

Dr. Draper, on a single occasion, thought that he 
obtained evidence, through photography, of a native 
emission of light from the Planet. It is concluded, 
however, that Jupiter sends forth no permanent light, 
though he may occasionally emit fitful gleams. 

The flattening at Jupiter's poles is about T 1 T . The 
Planet's most remarkable features are the dark and 
light belts running in a common direction across its 
disc. 

Red Spot — For many years a large spot upon 
Jupiter, of a brick-red color and elliptical form, 25,000 
miles long and 8,000 wide, awakened much interest in 
observers. It was situated just south of the great 
southern dark belt, and proved to be an atmospheric 
effect which has now almost entirely disappeared. The 
earliest record of this great spot's appearance is by 
Professor Pritchett, now of Glasgow, Mo. 

All the visible features of the Planet are atmos- 
pheric, and we never see its real surface. 

Jupiter's equator is but very slightly inclined to the 
ecliptic, and its orbit makes an angle of 1° 18' 41" with 
the earth's path in space. 



The Plaxets. 109 



Satellites. — Jupiter has four satellites, discovered in 
January, 1610. Io revolves around its primary in Id., 
ISh., 28m., and 36s., is distant from it 267,000 miles, and 
is 2,100 miles in diameter. Europa revolves around tlie 
primary in 3d., 13h., 17m., and 54s., is distant 425,000 
miles, and is 2,100 miles in diameter. Ganymede 
revolves in 7d., 3h., 39m., and 36s., is distant 678,000 
miles, and is 3,400 miles in diameter. Callisto revolves 
in 16d., 8k., 5m., and 7s., is distant 1,193,000 miles, and 
is 2,900 miles in diameter. 

The satellites revolve in orbits whose plane is almost 
in the plane of Jupiter's equator, and they frequently 
pass between the planet and the earth. The satellites 
are not equally brilliant, which has given rise to the 
opinion that they may be more or less obscured by dark 
spots. 

Transits. — During their transits across Jupiter's face, 
the first satellite appears to have a grayish tint; the 
second looks like a bright spot; the third as a dark 
brown spot; and the fourth appears nearly black. 

Have they Atmospheres? — The third and fourth satel- 
lites are thought to have no atmosphere, the first a very 
tenuous one, but the second, owing to its high reflection 
and the indistinctness of its shadow, is regarded as 
having a very dense atmosphere. 

The Flight of Light. — The velocity of light has been 
measured through, means of the eclipses of Jupiter's 
satellites. This velocity is computed at 185,000 miles 
a second. When the earth is near Jupiter the eclipse 
is noticed to occur sooner than the calculated average 
time ; and when the earth is far from Jupiter the 
eclipse is perceived to occur later than the predicted 
average time. This is because the light-flash is not 
instantaneous, but occupies time in its flight. 



110 A str ONOM v : Ne w a xd ld. 



Saturn follows Jupiter in the order of distance from 
the sun. Saturn is the outermost of the five Planets 
known to the ancients, and its orbit marked the frontier 
of the old system of worlds. 

Its C:lor. — Saturn appears to the unaided eye as a 
star of the first magnitude, and shines with a dingy, 
yellowish light. 

Its Year. — The Planet circles its great path in 
10,750.24 days, or in about 29J years. 

Orbital Velocity. — Its average velocity around its 
orbit is 5.9G miles a second. 

Apparent Motion. — Owing to its great distance its 
apparent motion among the stars is slow, passing over 
only about twelve degrees in a year. Still, its motion 
was well known to the ancients. 

Distance. — Its mean distance from the sun is about 
887,130,000 miles. 

Diameter. — Saturn's mean diameter is about 71,000 
miles. 

Shape. — Saturn is very much flattened at the poles, 
its equatorial being to its polar diameter in the ratio 
of about 10 to 9. This departure from the spherical 
shape is attributed to its high rate of rotary speed. It 
rotates with about twice the axial velocity of the earth, 
and makes a complete revolution in 10h., 14m., and 24s. 

Density. — Saturn has a lower specific gravity than 
even Jupiter. Its density is 0.75 that of water, and 
0.1325 that of the earth. . Thus it would easily float 
in water. 

Its Mass. — The mass of the Planet has been com- 
puted, from the control it exercises over its satellites, to 
be more than eighty times the earth's. 



The Plaxets. Ill 



Volume. — Its volume is about 700 times that of the 
earth. 

Internal Heat. — It is concluded, from its very low 
specific gravity, that Saturn still retains much of its 
primitive heat, and is, indeed, much hotter even than 
Jupiter. 

Belts. — Faint belts of delicate tints of brown and 
blue are visible on the globe of Saturn. These belts 
change their aspect from time to time, but, owing to the 
Planet's enormous distance, changes cannot be either 
well noticed or easily followed. 

Inclination of Orbit. — The orbit of Saturn is inclined 
to the ecliptic by 2° 29' 40", and departs a little more 
from the circular form than does that of Jupiter. The 
inclination of Saturn's axis to the plane of its path 
is about 63° 10' 32". The Planet's equator is much 
more oblique to the plane of its orbit than is the case 
with the earth. This obliquity amounts to about 31°, 
and so the Planet's seasons are much more varied than 
with us. 

Spectrum. — The spectrum of Saturn is essentially 
similar to that of Jupiter, showing the aqueous absorp- 
tion line, and the " red star" one. 

Saturn's Rings. — Saturn has a wonderful ring system, 
consisting of three main divisions. There is the gray 
outer ring, the middle bright ring, and the interior 
dusky or " crape" ring. Parts of the middle ring 
are brighter than the Planet. With a powerful tele- 
scope and a good position of Saturn a very faint 
division is noticed in the outer gray ring. The very 
best time to observe Saturn's rings is when the Pla- 
net is so situated that the plane of the rings, when 
produced, will pass between the earth and the sun. 
This occurrence, however ? is a rarity, and not until 



112 Astronomy: New and Old. 

VMM will the very best opportunity of this kind be 
offered. 

The Crape-Ring. — Professor Bond discovered the third, 
or crape-ring, in 1850. This crape-ring is somewhat 
transparent, the Planet being obscurely visible through 
it. 

Many observers are of the opinion that this inner 
ring has, since its discovery, been growing inward 
and becoming more and more visible, and, indeed, 
that the whole ring-system is gradually widening. 

Saturn not self-luminous. — The rings cast their shadow 
on the Planet, and the Planet on the rings, showing 
that both rings and Planet shine by reflected sun- 
light. 

Mechanism of Rings. — The safest mechanical opinion 
concerning the mechanism of the rings is that they 
consist of a vast school of small meteoric bodies revolv- 
ing in separate orbits around the primary. This satel- 
lite theory was first suggested by Roberval, in the 
seventeenth century; afterwards by Jacques Cassini, 
in 1715, and later still by Wright, in 1750. James 
Clerk Maxwell advocated it in 1857, in an essay which 
won the Adams Prize of that year. Advocates of the 
nelmlar hypothesis see in the Saturnian system an 
early stage in the development of all the planetary 
worlds. 

Inclination of Rings. — The plane of the rings is in- 
clined to the plane of Saturn's orbit by 27°. When 
the edge of the ring is turned eyewise or sunwards, 
owing to its great thinness, it is invisible except to 
the strongest glasses, and even with their aid appears 
only as a very fine line of light. 

Dimensions of Rings, — Guillemin gives the following 
table of the dimensions of the rings : 



The Plaxets. 113 



Diameter of outer ring 173,500 miles. 

Breadth of outer ring" 10,000 " 

Diameter of middle ring 150,000 " 

Breadth of middle ring 18,300 " 

Distance separating outer and middle ring 1,750 " 

Inner diameter of middle ring 113,400 u 

Breadth of crape-ring 9,000 " 

Distance of crape-ring from Planet. 10,150 " 

Entire breadth of ring-system 39,050 " 

Thickness of rings probably not more than 100 " 

The crape-ring joins the middle ring. 

Observers have disagreed more concerning the be- 
havior of Saturn and his system than about that of 
all the other Planets besides. Variations in the divi- 
sions of the outer ring, in the aspect of the crape-ring, 
and in the position of the ring-system have been 
repeatedly announced, and as often denied. The fluctu- 
ating character of our atmosphere, even when steadiest, 
is probably the chief cause of the discrepancies. The 
very high telescopic power required by the Saturnian 
system both gives the Planet a faint appearance, by 
spreading out its light, and magnifies the air-waviness, 
rendering the perception of delicate minutiae equivocal. 

Spectrum of Rings — The spectrum of the rings shows 
much less atmospheric effect than does that of the 
Planet. 

Satellites. — The following is a table of the elements 
of the eight satellites of Saturn : 

Mean Distance Sidereal Periodic 
Name. from Planet's Time. Discovery. 

Centre. D. H. M. s. 

Mimas- 118,000 miles 2 37 1-J.9 W. Herschel, Sept. 17, 1789. 

Enceladus 1*2,000 " ....128 53 6.8 W. Herschel, Aug. 28, 1789. 

Tethys 188,000 " 12118 26 J. D. Cassini. Mar. 21, 1684. 

Dione 241,000 " 2 17 41 9 J. D. Cassini. Mar. 21, 1684. 

Rhea 337,000 " 4 12 25 11 J. D. Cassini, Dec. 23, 1672. 

Titan 781.000 " . .15 22 4125 C. Huyghens, Mar 25,1655. 

Hyperion 946,000 " 21 7 7 41 Bond-Lassell, Sept. 19, 1848. 

Japetus 2,280,000 " .-,.-79 75340 J. D. Cassini, Oct, 25, 1671. 



114 Astronomy: New asd Old. 

The orbits of all these satellites, except that of 
Japetus, lie almost in the plane of the rings. The 
path of Japetas is inclined by about 8° 16' to this plane. 

Titan has been measured and its diameter computed 
at more than half that of the earth, and so its volume 
is about nine times that of the Moon. Titan is the 
largest of the satellites, and is plainly visible in a small 
glass. Japetas is also easily seen; but Rhea, Dione, 
and Tethys demand strong, and Mimas and Hyperion 
the most powerful, telescopes. Enceladus is not very 
difficult of observation. 

Japetus varies greatly in brightness. Sometimes it 
is 4J times brighter than at other times. It is thence 
concluded that this satellite turns the same face con- 
stantly toward the primary, and that one-half of its 
disc is dusky. 

URANUS. 

Uranus succeeds Saturn in the order of distance 
from the sun. This Planet was unknown to the 
ancients. 

Discovery — It was discovered by William Herschel 
on the night of the 13th of March, 1781, while examining 
some stars in the constellation of the Twins. 

Herschel was a keen observer, and as soon as the 
new Planet came into the field of view of his not very 
great telescope, he immediately discovered its minute 
disc, and knew that it was not a star. He at first 
thought it to be a comet; but the mathematicians 
Lexell and La Place soon announced the newly dis- 
covered body to be a Planet. 

Planetary Elements. — Its orbit is almost circular, and 
it traverses it in 30,688.39 days, or about 84 years. Its 
average distance from the sun is 1,784,000,000 miles. 



The Planets. 115 



Uranus shines as a star of the fifth magnitude, and 
is visible to the unaided eye. 

In the telescope Uranus shows a small sea-green 
disc. 

The diameter of the Planet is variously estimated at 
between 31,700 and 31,500 miles. Its volume is between 
G4 and 82 times that of the earth. 

Uranus is about 15 times as heavy as the earth, 
while the density of the matter composing it is only one- 
sixth of the earth's ; and so the Planet weighs very 
little more than an equal volume of ice. 

The heat and light received from the sun upon 
Uranus are about the 370th of what we receive. 

Its path is inclined to the ecliptic by 46' 21". 

Markings and Rotation. — Concerning the Planet's form, 
rotation, and markings there is little more than dis- 
putes. 

This is due to its immense distance and, conse- 
quently, small and faint disc in the very best telescopes. 
Young, Schiaparelli, and Schafarik have found a dis- 
tinct bulge upon the Planet, lying in the plane of the 
orbits of its satellites, and have concluded that the 
Planet rotates in this same plane. 

On the other hand, the markings on the Planet's 
disc would indicate that it rotates in a plane inclined 
at one-half a right angle to the plane of the paths 
of the satellites. 

Observations made at Paris in 1884 by the brothers 
Henry, and at Nice in the same year by Perrotin and 
Thollon, indicate that the direction of the Planet's 
rotation makes an angle of 40° with the plane of the 
orbits of the satellites. 

Buffiiain, in 1870-72, gave the time of rotation of 
the Planet as twelve hours. 



116 Astroxomy: New axd Old. 

The observations at Nice in 1881 indicate a rotation 
period of ten hours. 

These, however, must be regarded only as clever 
guesses. 

Spectrum. — The spectrum of Uranus is a remarkable 
one. The Fraunhofer lines of the solar spectrum are 
wanting in that of Uranus, and are replaced by six 
original absorption bands. One of these belongs to the 
blue-green line of hydrogen, and one to the " red-star 
1 i ii**-'^ The others are unknown. 

It is concluded from this spectrum that the atmos- 
phere of Uranus must be so highly heated as to be 
capable of reducing water to its elements of hydrogen 
and oxygen. 

Satellites— Uranus has four satellites whose existence 
is certainly known. William Herschel thought that 
he perceived at least two additional ones, but it seems 
that he was mistaken, and probably saw faint stars 
in the Planet's neighborhood. 

Elements of Satellites — W. F. Denning gives the 
following table of the elements of the satellites: 

Periodic Time Distance from 
Xames. around Primary. Planet. Discovery. 

P. H. M. S. 

Ariel 2 12 29 21 123.000 miles W. Lassell, Sept 14, 1847. 

TJnibriel 4 3 28 8 171,000 " O. Struve, Oct. 8, 1847. 

Titania 8 16 56 25. .. ..281,000 " W. Herschel, Jan. 11, 1787 

Oberon 13 11 6 55 376,000 " W. Herschel, Jan. 11, 1787. 

An Extraordinary Anomaly.— The satellites revolve 
in an orbit whose plane is almost perpendicular to the 
ecliptic, or Planefs path. This is an extraordinary 
anomaly in the solar system. The paths of all the 
Planets, great and small, and of the satellites, lie 
almost in the plane of the ecliptic ; and the motion of 
satellites and Planets is in the same direction, from 



The Planets. IVt 



west to east, or in a contrary one to the hands 
of a watch. The path of the Uranian satellites is 
turned away abont 98° from the ecliptic, and their 
motion is retrograde, or from east to west. 

The orbits of the Uranian satellites lie in the same 
plane, and their motion around the primary is the 
nearest approach to being circular of any other in the 
solar system. 

NEPTUNE. 

Neptune is the outermost of the known Planets. 

Discovery. — It was discovered through the pertur- 
bations it occasioned in the orbit of Uranus. Mathe- 
maticians, after making allowance for all the known 
elements in forming the path of Uranus, still found 
that the observed place of the Planet departed con- 
siderably from the theoretic one. These mysterious 
perturbations could only be attributed to the action of 
an undiscovered Planet, revolving in the awful void 
beyond Uranus. 

Two mathematicians, Leverrier and Adams, com 
puted independently the mass and place of the dis- 
turbing body. Dr. Galle, of Berlin, by direction of 
Leverrier, pointed his telescope to the heavens on 
the night of September 23, 1846, and found the Planet 
within 1° of the indicated place. 

It appeared, according to prediction, as an eighth 
magnitude star, with a tiny disc. 

It was one of the greatest achievements of mathe- 
matical astronomy, and has made the names of Le- 
verrier and Adams immortal. 

Planet's Color — The Planet's disc has a pale blue 
color, but owing to its great distance no markings 
have been discerned on it. 



118 Astronomy: New and Old. 

Planetary Elements — The diameter of Neptune is 
about 35,000 miles, or more than four times that of 
the earth. 

Its average distance from the sun is 2,795,000,000 
miles, or about 30 times the earth's mean distance. 

It travels in its orbit at the rate of about three miles 
a second, and requires 60,181.11 days, or about 165 
years, to complete a circuit of it. 

The inclination of its orbit to the sun's path is 1° 47'. 

The volume of Neptune is nearly 105 times the 
earth's, and its mass 21 times that of the earth. In 
reference to size, Neptune is the third Planet of the 
system . 

The density of Neptune is only one-fifth that of the 
earth. 

Heat and Light — The intensity of the heat and light 
received by Neptune is about the thousandth part 
of that received by us. 

Axial Rotation — Maxwell Hall, toward the end of 
1883, suggested that the Planet turned on its axis in a 
little less than eight hours. This, however, for want 
of data, is considered not even a good guess. 

Spectrum — Neptune gives the feeblest indications 
of a spectrum analogous to that of Uranus. 

Satellite — Neptune is accompanied by a single satel- 
lite, discovered by W. Lassell seventeen days after the 
Planet's discovery. The satellite revolves about its 
primary in about 5 days and 21 hours. 

Its mean distance from Neptune is about 220,000 miles. 

Motion Retrograde — The motion of this satellite 
around the primary is still more markedly retrograde 
than that even of those of Uranus. The satellite's 
orbit is almost turned upside down, its inclination to 
the ecliptic being 145° 7'. 



The Planets. 119 



If the motion of a single one of the great Planets 
had been retrograde, the destruction of our system of 
worlds would be a mere question of time. 

New Planets — It is strongly surmised by mathema- 
ticians that a Planet or Planets may exist beyond the 
path of Neptune. 

Professor Forbes, of Edinburgh, from his study of 
the conduct of certain comet groups, has formed the 
opinion that they are in some uu explained way con- 
nected with the Planets. Thus, for instance, a number 
of comet families have their point of greatest distance 
from the sun close to the orbit of Jupiter, and other 
groups are similarly situated with respect to Neptune. 

The professor has discovered that two large groups 
of comets have their aphelion points situated at dis- 
tances respectively of 100 and 300 times the radius 
of the earth's orbit. He surmises that Planets may 
revolve at these distances from the sun. 

Dr. Todd, of Washington, independently deduced 
from the "residual errors" of Uranus that Planets may 
revolve at these same precise distances. 



120 



Astronomy: New axd Old. 



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CHAPTER VIII. 

AEE THE PLANETS HABITABLE f 

There is scarcely a subject in Astronomy that opens 
a wilder field for speculation than this of the habitable- 
ness of the planets. In dealing with the question it 
will he safe to avoid dogmatism as much as possible. 

Conditions of Life.— It is very difficult to determine 
what are the conditions absolutely necessary for life on 
a planet. It was always thought in the past that no 
animal life could subsist in the great sea-depths, the 
pressure of the water being so great down there as to 
crush the mail of the crocodile. But lately it has been 
discovered that animals do live in these depths, and 
even have organs of vision. They have been brought 
to the surface by dredges, but had burst open long 
before they reached it. 

What We know of the Planets.— We must almost 
entirely judge of the constitution of the planets from 
their supposed analogy to the earth. And how lim- 
ited is our knowledge even of the physical constitution 
of the great body of our own planet ! We are actually 
acquainted only with the strata immediately contiguous 
to the earth's upper surface. 

What slight knowledge we possess of the interior 
is gleaned from a few fissures, or the bores and shafts 
of miners. None of these is more than a few thousand 
feet in depth. 

And yet we know even more of the interior of the 
planets than of their surface and climate. Their mass, 
density, weight, volume, and distance are being ascer- 
tained with ever-increasing exactness $ but, not with- 



122 Astroxomv: Xew axd Old. 

standing the constant use of our great telescopes, our 
knowledge of the physical character of a planet's soil 
and atmosphere is deeply obscure. 

Many circumstances that vitally influence life on the 
earth, snch as the chemical affinities of matter on its 
surface, the regular forms in which its molecules com- 
bine together, how the combinations and decompositions 
of the elements of matter are affected by light- waves, 
and the effects of electricity and electro-magnetism, 
remain to us, in regard to the planets, as the darkest 
of enigmas. 

It seems to be conceded that, in judging of the habit- 
ableness of a planet, its heat and light, the force of 
gravity 01 weight of bodies at its surface, and its den- 
sity or consistency, must be primarily considered. But 
we know that there are a multitude of circumstances 
besides of vital moment ; snch as the density and 
height of the atmosphere, the length of the day, the 
obliquity of the sun's rays. We must remember, too, 
that, as a supporter of animal life, the consideration ot 
the constitution of the atmosphere, and the proportion 
of deleterious gases mixed with it, are of paramount 
importance. 

Our atmosphere is a mixture of about one-fifth oxy- 
gen and four-fifths nitrogen. 

In certain countries, however, carbonic acid gas flows 
abundantly out of the ground, and vitiates the sweet 
air. In the island of Java there is a valley where the 
soil emits such quantities of this gas that nothing can 
live within its limits, and the birds that try to fly over 
it fall down and die. In the Pozzuolo grotto near 
Naples a man can walk without danger, but a dog can- 
not venture in without being immediately asphyxiated, 
owing to the inhalation of carbonic acid gas. This 



Are the Plaxets Habitable? 123 

deadly gas is heavier than air, and remains low down 
near the ground, so that the man escapes its influence 
while the dog inhales it with fatal consequences. 

What a multitude of circumstances, concerning 
whose existence we can hut conjecture, are to he 
regarded in judging of the habitableness of the plan- 
ets ! Moreover, every great discovery puts in hazard 
our formed opinions, or may force us to radically change 
them. The eighteenth century astronomers, guided by 
William Herschel, religiously believed that the sun was 
peopled with a superior race of beings. Herschel re- 
garded the sun as a large, lucid planet, having a cool, 
dark, solid globe, with a surface diversified by valleys 
and mountains, covered with luxuriant vegetation, and 
u richly stored with inhabitants." 

Overhead and surrounding this solid globe was a 
luminous solar aurora, thousands of miles in depth, 
sending forth light and heat. The surface of the great 
primary planet was protected from the intolerable heat 
and light by the intervention of a canopy of heavy clouds. 

This limitless Eden enjoyed a perennial spring. The 
lamp of Aladdin never guided happy genii to a more 
wondrous abode. 

The spectroscope absolutely dissipated this fabric of 
Herschel when it demonstrated to us the true physics 
of the sun. 

Outer Planets.— It seems to be the conclusion of 
astronomy and physics, as far as our present knowl- 
edge goes, that the four outer planets are still too high- 
ly heated to be in a fit condition for the maintenance 
of life such as we witness it on the earth. 

Their average density is very little more than that of 
water, Saturn's being even much less. It is, then, safe 
to say that they have no solid crust, 



124 Astronomy: New and Old. 

According to the testimony of the telescope, Jupiter's 
surface is undergoing continual changes. Heated va- 
pors are constantly rising from the interior, then, cooling 
down, are thrown back into the boiling mass beneath. 
The most awful tempests, incomparably wilder than our 
fiercest tornadoes, occnr in his glowing atmosphere. 
Jupiter seems to be, at present, in a middle state be- 
tween ;i planet and a sun. 

Saturn appears less heated than Jupiter, and its 
atmosphere calmer ; but it is as good as demonstrated 
that the planet has no solid surface. 

The telescope tells us little concerning Uranus and 
Neptune. The spectroscope seems to intimate that they 
have dense atmospheres. The little we do know about 
these planets shows them to be ill a similar condition to 
Jupiter and Saturn. 

Uranus receives from the hearth of the system only 
the 370th part of the light and heat that we do, and to 
an Dranite the sun would appear as a brilliant star, 
without perceptible disc. 

Minor Planet?. — Of the habitability of the minor 
planets but little need be said. Being so small, and 
entirely destitute of gaseous envelope, they cannot be 
the fitting abodes of life. 

Satellites.— We may, too, safely pass by the satellites 
of the great planets as not being capable of sustaining 
the life that we know. 

Our moon has neither atmosphere nor water, and 
is possessed of such an extraordinary climate that 
life us known to us could not survive a single hour 
upon it. 

Mercury.— Mercury has a solid crust like the earth, 
and its density is a little greater than that of our own 
planet. Its orbit being very eccentric, the light and 



Abe the Planets Habitable? 125 

heat it receives from tlie sim vary from four to ten times 
the amount reaching us. 

It rotates on its axis in about twenty-four hours, so 
that its day is about the same length as ours. 

The length of Mercury's year is only eighty-eight 
days, so that its seasons succeed each other very rap- 
idly. The time from midwinter to midsummer is only 
forty-four days, and in that short interval the heat of 
the sun increases from four to ten times the heat re- 
ceived by us. 

There is much doubt about the existence of an atmos- 
phere on Mercury, some contending that it has a dense 
one, and others, and perhaps the more trustworthy, 
vouching that it has none. 

It is certain that an atmosphere has much to do in 
modifying the effects of solar heat ; but it seems unrea- 
sonable to expect that any atmospheric envelope, how- 
soever dense, could protect life under the fearful fluctua- 
tions of the Mercurian temperature. 

The mass of Mercury is about the one-twenty-fourth 
of that of the earth, and a pound weight with us would 
be only seven ounces on Mercury. 

The axis of this planet is much inclined from a per- 
pendicular to its orbital plaue, and this adds greatly to 
the suddenness and violence of the vicissitudes of its 
temperature. 

Venus. — Venus has a dense atmosphere, heavily 
charged with moisture. This planet is similar to the 
earth in many particulars. Their size, mass, and den- 
sity are nearly equal. The length of their day is about 
the same. 

Venus, however, owing to its greater proximity to 
the central orb, receives twice as much heat and light 
as we do. 



126 Astronomy: New axd Old. 

The weight of bodies on its surface does not differ 
materially from what it would be upon the earth's 
si ui ace. 

There are, however, two respects in which Venus 
differs widely from the earth, and which have a vital 
effect on its habitability. These are, the length of its 
year, and the obliquity of its equator to its orbital path. 

The year of Venus is about 224 of our days, and its 
equator is inclined to its path by an angle of 53° 11' 26". 

The obliquity of the earth's equator to the ecliptic is 
only 23.]°. It is this obliquity that produces the seasons 
on the earth. If this obliquity had been greater, the 
change of seasons would be more decided ; and if less, 
the change would be less marked. 

Again, on the earth the seasons are divided by inter- 
vals of about ninety days, whereas on Venus, the year 
being 224 days, the seasons are separated by only fifty- 
six days. 

The Cythereau seasons must change with appalling 
suddenness and violence. To live through the vicis- 
situdes of this Cythereau climate would require a spe- 
cies of animals of iron constitution. 

Venus is, moreover, covered with a cloud-mantle, in 
which there is scarcely ever a rift, and which indicates a 
continuous rain-fall upon the planet. 

Under these circumstances, Venus may be the abode 
of an abundant and rich vegetation, but it is almost too 
much to expect that animals could survive a perpetual 
down-pour of rain. 

This seems to be the safest conclusion from our pre- 
sent meagre data. It must be said, too, that these data 
are shrouded in much uncertainty. The nature of the 
cloud canopy and the amount of equatorial obliquity 
are involved in some doubt. 



Abe the Plaxets Habitable? 127 

Mars.— Of all the planetary bodies Mars bears the 
closest analogy to the earth, and seems to give all tele- 
scopic and spectroscopic probabilities of conditions fa- 
vorable to life as we know it. And we are better 
acquainted with Mars than with any other planet, on 
account of its favorable position, and the freedom of 
its atmosphere from clouds and vapors. 

Still, these neighboring planets differ radically in a 
number of minor details which, taken together, might 
be fatal to the condition of life. 

The orbit of the planet is very eccentric. Mars is 
sometimes 26,000,000 miles nearer to the sun than 
at other times. The length of its year is 687 days. 
Its equator is inclined to its path by 27°, so that the 
change of its seasons is much more marked than 
with us. 

The mass of Mars is little more than an eighth of the 
earth's, and the force of gravity on its surface is less 
than one-half of what it is on the earth. 

Its atmosphere is much thinner than ours, and, 
instead of weighing fifteen pounds on the square inch, 
weighs only two and a quarter pounds. 

Its day is a little longer than ours. Its surface is 
about equally divided between land and water. 

Mars, as a world, unmistakably shows the signs of 
age. It appears much older than the earth. 

It is thought that, as in the case of the moon, the air 
and water have, in a great measure, been absorbed into 
the planet's interior. 

The continents of Mars everywhere interlace with 
arms of the sea, indicating that the land and water 
levels scarcely differ ; and Schiaparelli and others hold 
that their outlines are not constant. The encroach- 
ments of dusky upon bright tints suggest vast inunda- 



128 Astronomy: New and Old. 

fcions, occasioned by the melting during summer of the 
immense quantities of snow and ice gathered at the 
poles during the long winter, and completely deluging 
the entire surface of the planet. 

With its floods covering the whole surface, its atmos- 
phere rarer than that which oppresses the respiration of 
the traveller on the tops of our loftiest mountains, and 
its extreme variations of temperature, it is scarcely 
credible that man could live just now in the Martian 
world. 

Mars receives from the sun but between one-half and 
one-third the amount of light and heat that we do. 
Owing to this and to the peculiarity of its seasons, 
arising from the obliquity of its equator to the plane of 
its orbital path, the cold on this planet must be truly 
enormous. Linnaeus is of the opinion that the Martian 
year of 687 days is utterly destructive of vegetation as 
known to us. 

Whilst Venus looks to be a planet in its youth, Mars 
seems to have reached the sere and yellow leaf. 



CHAPTER IX. 
COMETS. 

Aspect,— We come now to treat of a wayward branch 
of the great solar family. The planets move around the 
sun in paths almost circular ; their orbits lie nearly in 
the same plane, and all move in the same direction. 

The cometary paths, on the contrary, depart widely 
from the circular form, are inclined at all angles to the 
ecliptic, and cometary motion is in every conceivable 
direction. 

Ordinarily a Comet consists of three parts : the nu- 
cleus, the coma, and the tail. The nucleus and coma 
compose the head of the Comet. The nucleus is the 
dense central part of the head that looks brightest to 
the eye. The coma is a fringe of foggy appearance sur- 
rounding the brilliant nucleus. 

The tail of the Comet is the luminous train stretching 
away from the nucleus. The tail is a continuation of 
the coma, is of a milky appearance, widens and grows 
faint as it recedes from the head, and invariably flows 
away from the sun. 

Some Comets are without tails ; others have no nu- 
cleus, and others again neither tail nor nucleus. These 
latter look like an irregular patch of vapor in the sky. 

Obey Gravitation.— Edmund Halley was the first to 
demonstrate that Comets obey at least one great law 
of the solar system, that of gravitation. 

Halley computed the orbit of the great Comet of 
1682, and predicted that it would return after intervals 
of about seventy-six years. 

By measuring the Comet's angular velocity, and 



130 Astronomy: New and Old. 

noting its place among the stars at different intervals, 
lie was able to trace its path, and compute the time con- 
sumed in traversing it. Having found the elements of 
its orbit, and the periodicity of its visits to the sun, 
Halley followed the Comet back to the birth of Mith- 
ridates, 130 B.C. 

Halley predicted that it would return in 1758, or the 
beginning of 1759. Clairant and Lalande calculated 
that the Comet would lie retarded one hundred days 
through the influence of Saturn, and five hundred and 
eighteen days through that of Jupiter, beyond the 
time of its return fixed by Halley. They accordingly 
placed the date of its nearest approach to the sun as the 
1 - > 1 1 1 of April, 1750, noting that their result might be 
inaccurate by a month either way. 

The Comet actually reached its perihelion on the 
12th of March, 1759. 

This computation must be regarded as a marvel of 
success when it is recollected that Uranus and Neptune 
were still undiscovered, and so no account taken of their 
perturbing effects on the Comet's orbit. 

Bailey's Comet is expected to reappear in the year 
1910. 

Planetary Comets -There are about a dozen Comets, 
besides Halley's, moving in elliptical orbits whose 
periods have been accurately ascertained. They are 
known as Periodic Comets, or "Comets of short period," 
and revolve around the sun from west to east similarly 
to the planets. 

Their orbits are not greatly inclined to the common 
track of the planets, and are much less eccentric than 
those of Comets generally. 

Encke's Comet, having a period of 3.3 years ; Biela's, 
G.G years ; Faye's, 7.4 years j Brorsen's, 5.6 years ; D'Ar- 



Comets. 131 



rest's, 6.4 years ; Winnecke's, 5.6 years ; Tuttle's, 13.8 
years ; Tempers, 6 years ; Halley's, 76 years, are the 
principal periodic or " planetary" Comets. 

All these, except Halley's, are telescopic Comets, 
invisible to the unaided eye, and remarkable for their 
short appendage and feeble nucleus. 

Comets are said to consume themselves in producing 
their tails. The substance ejected into the tail is lost to 
the central body. 

The sun causes the flow of matter from the head to 
the tail, and hence the more visits a Comet makes to 
the sun's neighborhood the more rapidly it wears out 
and is dissipated. 

The periodic Comets, consequently, owe their con- 
sumptive appearance to their frequent approach to 
the sun. 

The " planetary" Comets have their aphelion dis- 
tance in the vicinity of Jupiter or Saturn. Their desti- 
nies are controlled in great measure by these giant 
planets. It is surmised that these planets caught them 
on their first approach to the sun, and changed their 
orbits, making them periodic Comets, and may ulti- 
mately expel them from the system. 

It is very probable that a telescopic Comet of 1770 
had its orbit so radically changed by a close approach 
to Jupiter in 1779 as to be driven out into space. 

Resisting Medium— Encke's is the most celebrated of 
the planetary Comets on account of its connection with 
the theory of the existence of a resisting medium in 
space. 

Encke concluded that his Comet was gradually fall- 
ing upon the sun, owing to the presence of an extremely 
rare substance in the vicinity of the central luminary. 
He thought that while this medium influenced the 



132 Astronomy: New and Old. 

motion of such ethereal bodies as Comets, still it was 
altogether too immaterial to appreciably affect planet- 
ary velocity. Also, the medium was supposed to be 
confined to the immediate neighborhood of the sun. 

Encke calculated that his Comet traversed its orbit 
two and a half hours shorter each succeeding revolution. 

A resisting medium would have the effect of accele- 
rating instead of retarding a Comet's velocity, as it 
would shorten its path around the sun by throwing 
it in towards that luminary. 

Against the existence of the medium it was objected 
that other telescopic Comets were not in the least influ- 
enced by it. 

To this it was argued that Encke's approached much 
nearer to the sun than did the others, none of which 
encountered the medium. 

The great Comet, however, of 1882 upset altogether 
Encke's theory of a resisting medium. In its passage 
through the sun's surroundings the Comet experienced 
not the slightest resistance from any source whatever. 
It approached to within 300,000 miles of the solar sur- 
face, and the rare substance supposed to obstruct 
Eneke's Comet would be 2,000 times more dense at this 
Comet's nearest point to the sun than at the nearest 
point of Encke's ; and if the medium existed, its in- 
fluence would in this instance certainly be much more 
strongly felt. 

The track of this Comet was followed with accuracy 
for a week before its perihelion, as well as for months 
after it. The Comet's velocity before and after it 
reached perihelion was compared, and the computed 
place after perihelion perfectly agreed with the observed 
place, and thus demonstrated the non-existence of a 
resisting medium. 



Comets. 133 



Encke's Comet, August, 1835, being in the vicinity 
of Mercury, was utilized as a means of weighing the 
planet. The mass of Mercury was computed by Yon 
Asten, from its perturbing influence on the Comet's or- 
bit, to weigh about ^ of the earth's. 

The nebulous matter composing Comets contracts in 
bulk on an approach to the sun, and expands as they 
recede. 

Mass of Comets — The matter composing Comets is 
of extraordinary tenuity. The feeblest ray of light may 
traverse thousands of miles of cometary substance 
without perceptible diminution. As an indisputable 
instance of this, Dawes saw a star of the tenth magni- 
tude through the very centre of a Comet on the 11th 
of October, 1847. 

Cometary Tails not Permanent Appendages. — The tails 
of Comets are not really permanent appendages, but 
emanations, or inconceivably rapid outflows of exceed- 
ingly rare matter from the nucleus. This substance 
composing the tail is detached and lost to the Comet. 
The Comet's tail has been likened to the train of smoke 
flowing from the stack of a moving steamship, the form 
being preserved while the smoke is lost. 

On no other grounds can the perihelion sweep of 
a Comet's tail be accounted for, as no permanent append- 
age millions of miles in extent could be whirled around 
the sun in the short space of a few hours. 

Comets Short-lived — Comets appear to be but tran- 
sitory agglomerations subject to disintegration and 
final decay. This seems to be proven by the breaking 
up and disappearance of one (Biela's), and the gradual 
but sure waste of substance of almost all. 

Spectra of Comets. — Donati, in 1864, made the first 
successful application of the spectroscope to the exam- 
ination of Comets. 



134 Astronomy : New axd Old. 



He made the experiment on Tempel's Comet, and 
found its spectrum to consist of three bright bands of 
yellow, green, and blue divided by broader dark ones. 
Up to this time it was thought that comets shone 
entirely by reflected sunlight. This spectrum showed 
them to be chiefly composed of glowing gas, and self- 
luminous. 

Dr. Huggins found the cometary spectrum to be 
similar to that of olefiant gas (C 2 H 4 ). 

Eighteen Comets whose light has been examined in 
the spectroscope have given what is known as the 
hydro-carbon spectrum, due to the presence of acetylene 
gas (C 2 H 2 ). 

The brilliant comet of Coggia, discovered April 17, 
1874, was carefully examined by Yogel, Huggins, 
Bredichin, and Secchi. They unanimously pronounced 
its spectrum that of olefiant or marsh gas (CH 4 ). 

It had, owing to its brightness, the complete hydro- 
carbon spectrum, consisting of five bands of different 
colors. The telescopic Comets previously examined, 
owing to their feeble light, had given but three of these 
bands. 

Zollner has shown that, owing to evaporation and 
other changes caused by solar heat on the near ap- 
proach of Comets to the sun, strong electrical excite- 
ments are occasioned in these bodies. 

Comets, although principally composed of glowing 
gas, probably owe their light to electricity rather than 
incandescence. 

The spectrum proper of Comets is usually accom- 
panied by a faint continuous spectrum, due partly to 
reflected sunlight and partly to the heated glow of small 
solid particles. 

How Cometary Tails are Formed. — The train of a 
Comet is composed of matter in a condition of great 



Comets. 135 



tenuity. The particles of this matter are acted upon by 
a force of electrical repulsion from the sun. 

While the force of gravity of a body depends upon 
its inass, the electrical energy of attraction or repul- 
sion depends on the extent of its surface. As the 
size of the particles of matter diminishes, the efficacy 
of the sun's electrical repulsion, compared with its 
gravitational attraction, on a gathering of particles in- 
creases. 

If the particles become sufficiently small, the repul- 
sion will be greater than the attraction, and the par- 
ticles are repelled from the sun. 

It seems that the Comet and sun are similarly electri- 
fied, and while the denser nucleus obeys the laws of 
solar gravitational attraction, the much lighter particles 
emanating from it are electrically repelled, and form the 
train. 

To obtain this effect no greater degree of electrical 
excitement would be necessary in the Comet than is 
accorded to the earth's surface ordinarily. 

Classes of Comets — Professor Bredichin, of Moscow, 
divides Comets into three classes, for the individuals of 
each of which the repulsive force is the same. He 
studied the construction of thirty-six Comets, and found 
all to agree perfectly with his rule of division. 

In the first class the sun's repellent energy is twelve 
times as great as its attractive power. These Comets 
have great long, straight trains, projected away from 
the nucleus at the rate of 2.8 miles per second. To this 
class belonged Halley's, and the Comets of 1811, 1843, 
and 1861. 

In the second class the forces of attraction and repul- 
sion are equal on the average. The repulsion may ex- 
ceed the attraction 2 J times, or may descend j\ below 



136 Astronomy: New and Old. 

it. In this class the initial velocity of the matter form- 
ing the tail is 984 yards a second, and the tail has a 
plumy appearance. To this type belonged Donati's and 
Ooggia's < 'omets. 

In the third class the tails are formed with a repellent 
force of about one-fifth that of gravity, and the initial 
velocity of the train-matter is 328 yards a second. The 
tails of this type of Comets are strongly bent, short, and 
brush-like. 

These three classes of tails may be sometimes seen in 
a Single Comet. 

Bredichin found the substance composing the first 
class of trains to be hydrogen, the second hydro-carbon, 
and the third iron or some other metal. 

It would seem thai the atomic weights of these sub- 
stances are inversely proportioned to the repulsive 
force employed in the production of the respective 
trains. 

The truth of this theory of Bredichin lias been 
abundantly illustrated by the appearance of five recent 
bright Comets. 

Is Identity of Path Identity of Comets ? — Groups of 
Comets have been perceived pursuing one another, after 
intervals of several years, nearty in the same track, and 
consequently identity of path is no longer a certain test 
of the identity of Comets. 

Origin of Comets. — Laplace thought the ordinary form 
of a Comet's orbit, previous to planetary perturbation, 
to be that of a hyperbola, or an extremely open curve. 
According to this opinion Comets are foreign to the 
solar system, and are encountered by our system in its 
passage through space. 

Gauss and Schiaparelli, on the contrary, proved, in- 
dependently, that a Cornet's ordinary path is a very 



Comets. 137 



eccentric ellipse, the form of a hyperbola being the con- 
sequence of planetary disturbance. 

From this it is concluded, and it seems the better 
opinion, that before being attracted by the snn Comets 
formed a portion of the nebulous matter belonging to 
our system, sharing in its motion of translation through 
space, and so were relatively in a condition of repose. 
Comets are detached fragments of this general nebulous 
truck, which, straying within the circle of the sun's 
attraction, are drawn around him in an orbital path. 

Mass of Comets — The substance of a Comet's tail is 
of the most extreme tenuity, and its mass is only an 
affair of pounds or even ounces. 

It is not known whether the nucleus is a solid body 
or the most dense part of a meteoric cloud. Judging by 
analogy from our knowledge of telescopic Comets, it is 
probable that the nucleus is but a cloud of solid or 
liquid particles. 

Professor Peirce shows, however, that the Comets of 
1680, 1843, and 1858 must have had a nucleus of metallic 
density to have lived through the ordeal they sustained 
in their very close passage around the sun. 

From the fact that Comets have no appreciable effect 
in perturbing the planets it is evident that their mass 
must be utterly insignificant compared with the planet- 
ary masses. 

The chances of an encounter between the earth and 
the nucleus of a Comet are so infinitesimally small that 
they need not be considered. 

Remarkable Comets — The most brilliant Comet of 
this century was probably the one discovered on Feb- 
ruary 28, 1843. At its nearest approach to the sun but 
an interval of 32,000 miles separated their surfaces. 

The motion of the Comet at its perihelion reached 



138 Astronomy: New axd Old. 



300 miles a second. This awful velocity saved it from 
falling into the sun, and carried it through 180° of its 
orbit in the short space of two hours and eleven 
minutes. To travel over the other half of its orbit it 
may require hundreds of years. 

Its tail, which was slightly curved at its extremity, 
extended 40° along the sky. Its extent in miles has 
been computed at 51,000,000. This enormous appen- 
dage was whirled around from one side of the sun to the 
other in 131 minutes. 

Owing to the action of our atmosphere the Comet 
appeared to have confiscations swaying like a torch in 
the wind. 

Periods all the way from 6 to 533 years were assigned 
to it, The reason of the great discrepancy was the 
insufficiency of data available for making the estimate. 
Donati's— Giambattista Donati discovered the Comet 
called after him on the 2d of June, 1858. Its tail 
stretched at its best in a graceful curve over more than 
a third part of the visible heavens. Its extent in miles 
was 54,000,000. 

On September 17 a secondary tail was developed in 
the form of a straight, narrow ray, tangent to the curve 
of the primary, and extending to a still greater distance 
from the head. The visibility of the second tail con- 
tinued during three weeks, and a portion of the time it 
appeared duplicated. 

Bond estimated the diameter of the nucleus at less 
than 500 miles. 

Seven separate envelopes were detached from the 
nebulous mass surrounding the nucleus, and after rising 
successively toward the sun fell backwards to be assimi- 
lated with the train. 

On the 2d of October the nucleus outshone Arcturus, 
the second brightest gem of the northern heavens. 



Comets. 139 



The Cornet's orbit is a very eccentric ellipse, its peri- 
helion lying inside the orbit of Venus, and its aphelion 
distance is 5£ times the diameter of Neptune's orbit. 
It requires 2,000 years to traverse its path, and the 
Comet's motion is retrograde, or contrary to that of the 
planets. 

Comet of 1861 — The great comet of 1861 made its 
nearest approach to the sun on June 11 of that year. 
This Comet was noticeable for the number and com- 
plexity of the envelopes surrounding its head. 

Its brightness was inferior to that of Donati's, but 
its tail had an extraordinary length, stretching over an 
arc of the heavens of 118°. 

It makes a revolution of its elliptical orbit in 409 \ 
years. 

A most interesting incident connected with this 
Comet is that the earth passed through its tail on June 
30, 1861. On that occasion our planet was immersed in 
cometary matter to a depth of 300,000 miles. 

The collision produced not the slightest perceptible 
effect. 

Tebbutt's Comet—Mr. John Tebbutt, of New South 
Wales, discovered the Comet called by his name on the 
22d of May, 1881. This Comet passed the sun on the 
16th of June at a distance of about 68,000,000 of miles. 

On June 24, and a few nights following, it was a 
most brilliant object, and outshone every star in the sky. 

Its tail was plumed similar to Donati's, and it be- 
longed to the same type as that great Comet. 

The first successful attempt to measure cometary re- 
fraction was made through this Comet. 

It was the first Comet, too, of which a satisfactory 
photograph was obtained. The substitution of dry gela- 
tine for wet collodion, just introduced at that time, 



140 Astronomy: New axd Old. 

greatly increased the sensitiveness of the photogra- 
pher's plates. 

Janssen obtained a good photograph of the Comet 
from half an hour's exposure. 

Dr. Huggins succeeded by chemical process in ren- 
dering visible the invisible portion of the spectrum of 
this Comet, and the result still further established the 
presence of the carbon compounds in Comets. 

Comet of 1882. — A great Comet was discovered at 
Eio Janeiro, September 11, 1882. The Comet attained 
such brilliancy on the 18th of September that it could 
be seen in day-time in a clear sky all over the world. 

It was the brightest Comet since 1843, and was 
visible to the unaided eye in daylight during three 
days. 

The course of this Comet was diligently watched by 
astronomers, who followed it with their telescopes to a 
distance of 470,000,000 of miles from the earth. 

It moves in a very elongated ellipse, and traverses its 
orbit in about S43 years. Its tail reached to a distance 
of 200,000,000 of miles from the head. 

Alter its passage around the sun the Comet gave 
evidence of breaking up. Its nucleus separated into 
five parts, and in its retreat from the sun the Comet ap- 
peared to leave fragments all along its track. 

August and July seem to be the months of the year 
most prolific in Comets, and May has on the average 
the least number. 

The majority of Comet discoveries have been made 
in the vicinity of the sun ; and this is but reasonable, 
since the sun is the focus of cometary motions. The 
western horizon, after sundown, and the eastern horizon, 
before dawn, are the most likely places to discover 
Comets. 



CHAPTER X. 
SHOOTING-STABS. 

Cause of the Phenomena. — Myriads upon myriads of 
particles of loose matter are circling within the plane- 
tary spaces in regular orbits around the sun. These 
particles are moving in every conceivable direction, 
but nearly always in the path of a comet. Indeed, 
they are regarded by astronomers as cometary debris. 

The earth sweeps along its path at an average rate 
of eighteen miles a second, and in its rapid progress 
is incessantly encountering these flying particles. 

The velocity with which the particles enter our at- 
mosphere is the resultant of their own and the earth's 
motion. When moving in opposite directions this re- 
sultant is the sum, and in the same direction the dif- 
ference, of their respective velocities. 

The average speed with which these tiny bodies 
strike the higher portions of our air is computed at 
thirty-five miles a second. 

The very rarest region of our upper atmosphere offers 
enormous resistance to bodies travelling through it with 
this awful rapidity, and the friction of their passage 
generates an intense degree of heat. 

This glowing heat renders the particles luminous 
while it consumes them. The flight of the burning par- 
ticles through the air occasions the phenomena of 
Shooting-Stars or Meteors. 

Heat of Meteors. — The amount of heat produced by 
the atmospheric resistance has been calculated on me- 
chanical principles. It is found by experiment that the 
temperature of a body moving through the air rises one 



142 Astronomy: New axd Old. 

degree Fahrenheit when the rate of speed is one hun- 
dred and twenty-five feet a second. Friction develops 
heat in bodies passing through the air in proportion to 
the square of the velocity. 

Hence, particles striking the atmosphere with a 
velocity of thirty -five miles a second are heated beyond 
two million degrees Fahrenheit. 

Are Shooting-Stars Dangerous ? — The earth encounters 
annually millions upon millions of these tiny particles, 
and tiny as they are, still their prodigious speed gives 
them the momentum of a cannon-ball. The atmosphere, 
however, consumes them, and thus actually affords us a 
better protection against them than could a shield of 
steel armor many feet in thickness. 

These cosmic atoms become luminous at an average 
atmospheric height of seventy miles, and are volatilized 
or completely dissipated into vapor before descending 
to a height of fifty miles. 

Meteoric Showers. — Halley was the first to hold the 
opinion that Shooting-Stars have a cosmical origin. 

Ohladni, in 1794, taught that space is filled with 
small circulating atoms which, becoming ignited by the 
friction of their rapid passage through our atmosphere, 
appear as Shooting-Stars. 

Occasionally Shooting-Stars fall in enormous showers 
and produce thrilling spectacles. The particles them- 
selves through whose combustion in the air the meteor 
showers are produced are called meteoroids. 

The Leonids of November. — A great shower of Shoot- 
ing-Stars fell upon the earth on the night of November 
12, 1833. They fell in an incessant tempest for the 
space of nine hours, visible to the whole of North 
America, and with a frequency computed at half that 
of the flakes in a snow-storm. 



Shoottxg-Stars. 143 



The meteors all appeared to come from the same 
point in the heavens, the star Mu, in the constellation 
of the Lion. 

This star is called the radiant, or vanishing point, 
of these November meteors, because they appeared to 
all spring from the particular part of the sky occu- 
pied by the star, and thence flew off in every direction. 

The earth's course just then was straight toward 
Leo, and the meteors entered the atmosphere from the 
direction of the star Mu. And as Leo, with the advance 
of night, travelled up the sky from east to west, so 
the radiant point of the meteors travelled westwardly 
with it. 

The great November shower is known as the U Leo- 
nids," from having its radiant in the constellation of 
Leo. 

Path of the Leonids. — This great November shower 
has a periodicity. Indeed, the meteoroids were found 
by Olmstead, Olbers, Schiaparelli, Newton, and Adams 
to be travelling in an elliptical orbit around the sun, 
having their aphelion distance beyond the orbit of 
Uranus, and crossing the earth's orbit near their 
perihelion. 

On the night of November 12 the earth crosses the 
path of the meteoroids. 

The meteoric shoal completes a revolution of its orbit 
in 33.27 years. Consequently, about three times in a 
century the great swarm crosses the track of the earth. 

History of the Leonids — Professor Newton, of Yale 
College, traced the November meteors, or Leonids, back 
to the year 902 A.D., the date of their first recorded 
appearance. The shower has since reappeared twenty- 
nine times. 

He predicted their return on the night of November 



144 Astronomy: New axd Old. 

13, 1866. His prediction was accurately fulfilled; the 
shower appearing this time, however, in Europe instead 
of America. 

They appeared in America in November of the year 
following, 1867, the American hemisphere being then 
in front. 

These splendid displays drew the attention of astron- 
omers very forcibly to the subject of meteors. 

Schiaparelli on Meteoroids. — Schiaparelli gave as the 
result of his observations that meteoroids travel around 
the sun in very eccentric orbits resembling those of 
comets; that meteoroids move in all possible directions, 
and their orbits are at all inclinations to the plane of the 
ecliptic ; and that, similarly as comets, meteoroids have 
been drawn into our neighborhood by the sun, and 
frequently held there by the attraction of some great 
planet. 

The Perseids —Schiaparelli first announced that the 
August meteoroids travel in the same path with Comet 
III., 186:2. The great August showers fall between the 
9th and 11th of the month, and are called the " Per- 
seids," from the fact that their radiant is situated in the 
constellation of Perseus. 

In 1867 Leverrier, Peters, Adams, and other mathe- 
maticians demonstrated that the path of the November 
meteoroids corresponds exactly with the orbit of Tern- 
pel's comet, or, more technically, Comet I., 1866. 

The meteoroids follow precisely in the wake of the 
comet. 

The Lyrids. — About the same time Weiss, of Vienna, 
showed the identity of the path of Comet I., 1861, with 
that of a meteoric shower recurring on April 20, and 
known as the " Lyrids." 

The Andromedes. — He also demonstrated that the 



Shootixg-Stars. 145 



" Androinedes," or ineteoroids of November 27, are 
travelling in the track of Biela's comet, or Comet III., 
1852. 

There are other showers travelling in the paths of 
comets, such as the Geminids, Orionids, etc. 

Professor Alexander S. Herschel has shown that the 
orbits of seventy-six meteoric showers coincide with 
those of comets. 

Three Great Showers. — There are, however, only three 
great showers : the Leonids, Andromedes, and Perseids. 

Tempel's Comet — Tempel's is a telescopic comet, sup- 
posed to have been captured by the planet Uranus, and 
brought into our system in the year 126 of the Christian 
era. 

The meteoric or waste material following in the path 
of this comet is spread out over about one-tenth of the 
whole orbit. 

When Uranus caught the swarm it changed the 
speed of the particles by attracting them unequally, 
drawing the near ones more forcibly than the more 
distant. Accordingly, in time, owing to their different 
velocities, the particles will extend along the whole 
orbit, and we will have displays of equal splendor every 
12th of November. 

This is actually the case as regards the meteors of 
the 9th of August, or the Perseids. 

On every 9th of August there is an equally remark- 
able meteoric shower. 

We cannot Escape the Leonids. — The great November 
swarm is so extensive that, though travelling at the rate 
of twenty-six miles a second, it requires more than a 
year to pass a single point of its orbit, and on its suc- 
cessive visits to our vicinity can never escape encoun- 
tering the earth ; and so we must necessarily have the 



146 Astronomy: New axd Old. 

superb showers of the Leonids at intervals of 33.27 
years. 

Comet III., 1862. — The comet to which the Perseids 
belong, Comet III., 1862, is supposed to have been much 
longer in our system than Tempel's, and, accordingly, 
the particles following it are spread out along the whole 
length of the orbit. 

Biela's Comet.— In 1867 Weiss, D' Arrest, and Galle 
demonstrated that Biela's comet had broken up into 
the meteoric swarm of November 27, or the Andro- 
medes. 

The Andromedes have their radiant in the star 
Gamma, of the constellation of Andromeda. 

The Speed of Meteors. — The Leonids move in a direc- 
tion diametrically opposite to the earth's course, and so 
we rush to meet them. 

They travel with a rapidity of twenty-six miles a 
second, and, the earth's velocity being eighteen miles, 
the Leonids encounter our atmosphere at a rate of forty- 
four miles a second. 

The Androinedes, on the contrary, journey in the 
same direction as the earth, but with much greater 
speed, and so overtake us, striking the air with a 
velocity of twelve miles a second. The Leonids, con- 
sequently, make a much more magnificent spectacle 
than the Androinedes. 

None of the particles of the great showers have ever 
been noticed to reach the earth's surface. It is probable 
that the largest of them do not exceed in size a canary- 
seed, and are all entirely consumed at an average 
height of fifty miles. 

Fireballs or Bolides. — Fireballs, or bolides, are classed 
with meteors, and distinguished from the ordinary 
shower only by their greater size. 



Sro o ting- Stars. 147 



The fireball has an apparent diameter, while the 
common meteor is but a luminous point. 

Fireballs have long, luminous trains, a slow motion 
comparatively, and occasionally explode with a loud 
detonation. 

Fireballs are rare, a few of them usually accom- 
panying the great shoAYers. They appear, too, occa- 
sionally apart from the showers. 

A Brilliant Fireball. — W. F. Denning saw, on the 
night of August 13, 1888 (11 h., 33 m., G. M. T.), a 
brilliant fireball, furnished by the Perseid shower. It 
flashed like vivid lightning, and left a streak for three 
minutes. 

This great fireball appeared over Yorkshire, and fell 
from the height of seventy-eight to that of forty-seven 
miles. Its luminous train was eighteen miles long, 
and appeared at heights of fifty-nine to forty-seven 
miles. 

Meteorolites or Meteorites. — Meteorites are the solid 
masses that fall to the earth's surface from space, and 
are usually either stone-falls or iron-falls. 

Stone-falls. — The stone-falls have been frequent since 
the first authentic shower that fell, as told by Livy, on 
Mount Alban, near Rome, in the year 654 B.C. 

Taking the whole earth, the annual stone-falls may 
be counted by the hundreds. 

Iron-falls. — The iron-falls are much rarer than the 
stone-falls. 

Stone-falls are called aerolites, and iron-falls aerosid- 
erites or siderites. 

Composition. — The elements of both stone-falls and 
iron-falls are not unknown to the earth, and we are 
familiar, too, with the manner in which they are found 
chemically combined in these masses. 



148 Astronomy: New and Old. 



Stony meteorites, though containing many ingre- 
dients, may be said to be principally composed of nickel, 
cobalt, iron, and phosphorus. 

The chief constituent of the iron meteorites is an 
alloy of iron and nickel. Masses of this alloy, exactly 
as it is found in the siderite, have been vomited from 
the earth's interior. 

Aerosiderolites.— Besides the stone-falls and iron-falls, 
however, there are other types of meteorites of an inter- 
mediate structure and called aerosiderolites. 

Behavior and Aspect.— The fall of the meteorites is 
accompanied by a sound much louder than the most 
violent thunder. 

These bodies do not fall vertically, but at long 
range, and during the night hours are intensely lumi- 
nous. 

Meteorites are in nowise connected with comets. 

Structure.— They are usually of a rounded form or 
appear as fragments of what was primitively round. 

The meteorite is mostly covered with a glaze, formed 
by the fusion of its own substance. The heat occasioned 
by atmospheric friction produces this fusion. 

Meteorites range in size from ounces to tons. When 
meteorites strike the denser portion of the air they 
generally explode and fall to the earth in a small shower 
of fragments. 

Cause of Explosion — These bodies come from the 
region beyond our atmosphere, the average temperature 
of which is estimated at 400° Fahrenheit below zero 
(mercury freezes at 39° below). 

While the surface of the meteorite is intensely heated, 
the interior is cold beyond conception. 

It is probably due partly to this difference in temper- 
ature, and partly to the concussion of the air caused by 



Shooting-Stars. 149 

their rapid motion, that the meteorites so frequently 
explode. 

Origin of Meteorites. — The little that can be said 
concerning the origin of meteorites is of the most un- 
certain character. Every theory that has yet been pro- 
posed is fraught with almost insuperable difficulties. 
The nioon, planets, planetoids, the sun, interstellar 
space, and the earth itself have been variously sug- 
gested as their source. 

The Terrestrial Theory. — Those who attribute their 
parentage to the earth contend, and justly, that bodies 
expelled from terrestrial volcanoes with a velocity suffi- 
cient to carry them beyond its attraction would circle 
around the sun in orbits necessarily intersecting our 
planet's path at their point of first departure. 

It would often happen that the earth would just 
arrive at that part of its orbit when these bodies were 
careering by, and so encounter and catch them. 

A Difficulty. — To drive a body beyond the sphere of 
its attraction the earth would have to impart to it an 
initial velocity of six miles a second. This is called the 
critical velocity for the earth, and is so entirely enor- 
mous that we have no record of volcanic force capable 
of producing even an approach to it, although the ex- 
plosion which occurred at Krakatao, at five minutes 
past ten on the 27th of August, 1883, was heard, ac- 
cording to official evidence, at a distance of eighteen 
hundred miles, and the air-wave injured buildings two 
hundred miles away. 

In answer to the difficulty, however, the advocates 
of the terrestrial origin of meteorites say that the earth 
when younger had much greater volcanic vigor than 
it now possesses. 

Mineralogists seem to regard their origin as volcanic j 



150 



Astronomy : New and Old. 



l>nt where the volcanoes are located is the kernel of the 
difficulty. 

The six months, July to December, have two and one- 
half times more meteoric apparitions than the six 
months from January to June. 

Meteors are usually more numerous in autumn than 
in spring, and in the morning than in the evening 
hours. 

The hourly average is about 12 meteors for the whole 
year. Before midnight the hourly average is 9, and 
15 in the morning. 

The highest rate of apparition is reached about three 
o'clock A.M. 

Telescopic meteors are much more abundant than 
those visible to the unaided eye. The proportion is 
stated on good authority to be as high as forty to one. 

LIST OF PRINCIPAL METEORIC SHOWERS 
VISIBLE DURING THE YEAR. 



Time of Prin- Approxi- 


Notes. 


cipal Display. 


mate Star. 


Januarv 1-4. - - - 


fi Bootes 


Annual. 


March 3-5 


(5 Leonis 


Large meteors. 


April 17-20 .... 


a Lyrae 


Comet I., 1861. 


April29-May2. 


a Aquarii .... 


Fine meteors. 


July 23-25 


ft Persei 


Probably Comet of 1764. 


August 9-11 . . . 


rf Persei 


Annual. 


September 4-9 . 


s Persei 


Fine display. 


October 17-20.. 


v Orionis 


Annual. 


November 1-5 . . 


e Arietis 




November 12-14. 


y Leonis 


Annual: Comet I., 1866. 


November 27 . . . 


^Andromedse. Annual: Biela's Comet. 


December 8-13. 


a Geminorum . Annual. 



CHAPTER XL 
THE ZODIACAL LIGHT. 

Aspect — The Zodiacal Light is the great lens-shaped 
mass of light which appears in the West in the even- 
ing, shortly after sunset ; and in the East in the 
morning, before dawn. 

It is of a pearly glow, and is seen to spread over the 
part of the sky in the immediate vicinity of the point 
where the sun has disappeared, or is about to reappear. 
It is brightest at the centre, and its lustre gradually 
fades toward the exterior. Indeed, the light from about 
the edges is so feeble that it is frequently difficult to 
detect where it begins, and, consequently, the limits of 
this spindle of light seem irregular and poorly denned. 

The sun is situated at the centre of this lenticular 
spectacle. 

The phenomenon is still an enigma, and its conduct 
at times is very mysterious. 

It is not Twilight. — It can be easily distinguished 
from twilight in this, that the latter extends broadly 
along the horizon and reaches but a short distance up 
the sky. 

The Zodiacal Light, on the contrary, is a narrow 
beam, steeply inclined to the horizon, and extending up 
the heavens as high ordinarily as 50°, and rarely even 
as 100 p . 

Whence its Name — The beam is usually inclined to 
the horizon at an angle of 60° or 70° in our latitude, 
and is directed along the zodiac toward a point south- 
west of the zenith ; and hence its name of Zodiacal 
Light. 



152 Astroxomy: New axd Old. 

Arago and others were of the opinion that its color, 
especially toward the base, is inclined to yellow or red. 

Home observers say that the direction of the axis of 
this pyramid of light coincides witli the solar equator; 
and others, with the plane of the earth's orbit, or 
ecliptic. 

Its Changing Aspect — The apparent place of the light 
varies with the inclination of the ecliptic to the horizon, 
and its apparent lustre depends on the season. 

Its light is brightest when the ecliptic is most ob- 
lique, so that in northern latitudes it is most favorably 
seen in the evening in February and March, and in the 
morning in September and October. 

Its Lustre — The Zodiacal Light, in its brightest 
parts, far surpasses in lustre the most brilliant por- 
tions of the Milky Way. 

Toward the Tropics it increases in intensity and 
height, and is visible all the year round. 

Humboldt says that its lustre in Mexico and on the 
table-lands of Quito is very remarkable, and far ex- 
ceeds even that of the portion of the Galaxy between 
the Eagle and Swan. 

Zodiacal Cone. — This light is also called the Zodiacal 
Cone, from its triangular, pyramidal, or conical form, 
and to distinguish it, too, from the Zodiacal Band and 
the Gegenschein, or Counter Glow. 

Zodiacal Band. — The Zodiacal Band is much feebler 
than the Zodiacal Cone, of which it is a continuation, 
and extends along the whole zodiacal road from horizon 
to horizon. 

The Zodiacal Band is one of the faintest objects in 
the heavens ; its lustre is much below the dimmest por- 
tions of the Milky Way, and demands the keenest 
vision to discern it. 



The Zodiacal Light. 153 

This band grows dimmer as it advances up the sky 
away from the sun, until the opposite horizon is almost 
attained, when its lustre again greatly increases. 

Gegenschein, or Counter Glow. — This bright part of the 
Zodiacal Band, near the horizon on the side of the 
heavens away from the sun's place, is called the Gegen- 
schein, or Counter Glow. 

Not Atmospheric. — The Zodiacal Light cannot belong 
to our atmosphere, since it shares in the diurnal motion 
of the heavens, is visible in regions of the earth 
widely apart, and is almost invariably inclined along 
the ecliptic. 

Not a Solar Atmosphere. — It cannot be a solar atmos- 
phere, since Laplace has demonstrated mathematically 
that no envelope sharing in the sun's axial rotation can 
extend farther from his surface than -^ of the mean 
distance of Mercury, and this lenticular light reaches 
out sometimes even beyond the earth's orbit. 

What is It ? — Childrey was the first to observe this 
phenomenon, between 1658 and 1601 ; and Dominicus 
Cassini, 1683, to determine its relations in space. 

Humboldt was of the opinion that the phenomenon is 
probably due to the existence of a very compressed 
annulus of nebulous matter, revolving freely in space 
between the orbits of Venus and Mars, and capable of 
reflecting sunlight. 

Laplace considered it a zone of vapor, thrown off by 
the sun when in process of condensation from its primi- 
tive condition to its present viscous consistency. 

Other astronomers have looked upon it as an exten- 
sion of the solar corona. 

Others, again, as a flattened nebulous ring, surround- 
ing the sun at some distance. 

Against this it is argued that the distance from the 



154 Astronomy: New and Old. 

summit of the cone to its base is frequently observed to 
change. This change, however, in case the phenomenon 
is regarded as a luminous ring, may be accounted for 
by supposing its form to be elliptical, or, if circular, 
that the sun is not in the centre. 

Most recent Theory — The most recent theory con- 
cerning this luminous spindle is that it is composed 
of cosmical dust, planetary refuse, cometary waste- 
material, and ejected vapors, circling around the sun 
and reflecting sunlight. 

It seems to be the conclusion of science that the vast 
swarms of meteoroids circulating around the sun are 
certainly connected with the phenomenon. 

The great objection against the meteoroid theory is 
that this Light is much more dense in the plane of the 
ecliptic than elsewhere ; and as meteoroids travel in all 
possible directions, there would be no sufficient reason 
for this increase of density. 

It is also objected that there should be more variety 
than there actually is in the brightness and form of the 
light if it were produced by circulating meteoroids. 

On the other hand, the presence of the Gegenschein 
would seem to favor the theory that this light is due to 
the reflection of meteoroids, as these particles would 
become more luminous when in opposition to the sun. 

Professor Wright found that the spectrum of the 
phenomenon was probably that of reflected sunlight; 
and this would certainly favor the meteoroid hypothesis. 

Professor Lewis, too, made its spectrum probably 
that of reflected sunlight. 

It is, indeed, generally conceded that the spectrum 
of the Zodiacal Light is that of reflected sunlight. 

The reason that there seems to be so much doubt 
about this spectrum is owing to the faintness of the 



The Zodiacal Light. 



155 



light itself. A very wide slit must necessarily be em- 
ployed to catch a sufficient amount of lustre to* give a 
visible spectrum ; aud a wide slit, with the best light, 
impairs the definition. 

It can be demonstrated that a vast swarm of parti- 
cles circulating near the sun would so reflect his light 
as to give the appearance of the Zodiacal Light. 

THE GREEK ALPHABET. 



A 


a 


Alpha 


N 


V 


Nil 


B 


fi 


Beta 


3 


z 


Xi 


r 


r 


Gamma 








Omicron 


A 


6 


Delta 


n 


7t 


Pi 


E 


€ 


Epsilon 


p 


P 


Eho 


Z 


2 


Zeta 


2 


s 


Sigma 


H 


V 


Eta 


T 


r 


Tau 


& 


S 


Theta 


T 


V 


Upsilon 


I 


l 


Iota 


$ 


9 


Phi 


K 


K 


Kappa 


X 


X 


Chi 


A 


X 


Lambda 


w 


* 


Psi 


M 


n 


Mu 


a 


GO 


Omega 



CHAPTER XII. 

THE STARRY HEAVENS. 

I.— -THE CONSTELLATIONS. 

Their Antiquity—From a very early antiquity the 
stars have been divided into groups or catasterisms, 
called Constellations. 

Ancient astronomers called the Constellations and 
brightest stars after their national gods and heroes 
occasionally giving them, too, the names of animals. 

The most ancient reliable record, the Book of Job 
mentions the Pleiades, Orion, and Arcturus. Homer 
alludes to the Great Bear, Bootes, Pleiades, and 
Orion. 

Humboldt, however, is of the opinion that in the 
Greek sphere the stars were only gradually arranged 
m Constellations, since all the Greek names of stars 
and catasterisms were not of equal antiquity. 

Modern Catalogues.— Among modems, Bayer, in 1654 
established a sidereal nomenclature, retaining the 
ancient names of the Constellations, and designating the 
individual stars according to the order of brightness 
by the letters of the Greek alphabet. When the Greek 
became exhausted he had recourse to the Boman 
alphabet. 

As a rule the brightest star of a Constellation is 
called Alpha ; still there are exceptions to this, the 
chief star being not unfrequently designated by Beta 

His method was to write the Greek letter of the star 
before the genitive of the Latin name of the Constella- 
tion. In this way Sirius is written in the catalogue 
as a Cams Majoris, and Vega, a Lyra3. 

156 



The Starry Heavens. 157 

Flamsteed, in 1700, introduced a method of nomen- 
clature in which the individuals of each Constellation 
are represented by numbers in the order of their right 
ascension. 

Polaris [a Ursce 2Tinoris). — The daily eastward rota- 
tion of the earth on its axis gives the stars an apparent 
motion toward the west. The whole celestial sphere 
seems to revolve daily from east to west about one point 
in the north, called the Pole. 

Around this point as a fixed centre all the heavenly 
bodies move in circles, whose sizes depend on their 
respective distances therefrom. 

There is no star situated precisely at this pole. How- 
ever, a bright lone star of the second magnitude is in its 
immediate neighborhood, being distant from it less 
than li°. 

This star is called Polaris, and its apparent diurnal 
motion is so small that to the unaided eye it appears 
stationary. 

Polaris is easily discerned, there being no star of 
equal lustre within quite a distance of it. It is still 
more strongly marked by two stars in the Dipper point- 
ing constantly almost directly toward it. 

Ursa Major (Great Bear). — This Constellation is also 
known under the names of Dipper, Plough, and Charles' 
Wain. 

It consists of seven chief stars, six of them being of 
the second magnitude, and one of the fourth. 

Four of the stars, in the figure of a quadrilateral, 
form the cup of the dipper, and the three others the 
handle. 

The two stars in the cup, on the side opposite to the 
handle, point toward Polaris, and are on this account 
called the " pointers," 



158 Astronomy: New and Old. 

A line joining the pointers, and continued north 
about four and one-half times their own distance apart, 
will almost touch Polaris. 

The Great Bear has one hundred and thirty-eight 
stars visible to the unaided eye. 

Cassiopeia. — This Constellation is almost directly 
across from the Great Bear, on the opposite side of 
Polaris. 

These two Constellations are almost equally distant 
from Polaris. 

The six principal stars in Cassiopeia form the appear- 
ance of a W, or reversed chair. 

The Constellation has sixty-seven stars visible to the 
unassisted eye, two of them being of the second mag- 
nitude. 

Ursa Minor (Little Bear J. — This Constellation is situ- 
ated between Cassiopeia and the Great Bear, and con- 
tains twenty-seven stars visible to the unaided eye. 
Polaris is its brightest gem. 

Leo (The Lion). — The Lion is one of the conspicuous 
figures of the heavens, and is easily distinguished by six 
stars, that mark out the form of a sickle standing 
upright on its handle. 

Kegulus is the large white star in the bottom of the 
handle. 

Denebola ( fi LeonisJ shines in the eastern border of 
Leo. This star is of a bluish tinge, and almost as bright 
as Begulus. 

Leo has about seventy-five stars visible to the unas- 
sisted eye, three of them being of the second magnitude. 

The Sickle is almost directly overhead, or in the 
zenith, at 8 o'clock p.m., May lj 10 o'clock P.M., April 
lj 12 o'clock A.M., March 1; 2 o'clock A.M., February 
lj and 4 o'clock a.m. ? January 1, 



The Staff y He avexs. 159 

Bootes. — ^orth-east of the Sickle, and in an extension 
of the curve formed by the handle of the Dipper, is 
found the very bright yellow star Arctiirns. 

This is the great brilliant that marks the Constella- 
tion of Bootes. 

Bootes has eighty-five stars visible to the unaided 
eye. 

Cancer (The Crab). — This Constellation lies due west 
but a short distance from the Sickle, and is distinguished 
by a bright patch situated between two small stars. 

This patch is known as the Prsesepe, or Manger. The 
two dim stars on either end are called Aselli, or the 
Ass's Colts. 

This silver spot in the sky is also called the Bee- 
hive. 

The Manger is a weather prophet. When clearly 
visible it presages fine weather, but when it fades from 
view suddenly after unusual lustre the storm king is 
coming. 

Corona Borealis (Northern Croivn). — This is a very 
conspicuous object in the sky, and consists of six stars 
in the form of a half- circle or crown. 

It is but a short distance from, and almost due east 
of, Arcturus. Its brightest star is called Alpheta. 

Orion — Orion is certainly the most superb Constella- 
tion in the northern heavens. Four of its chief stars 
form a great quadrilateral, having in its centre, in 
a straight line and close together, three second-magni- 
tude stars, forming the Hunter's Belt. 

Two of Orion's gems are very brilliant ones 
indeed. These are Betelgeuse (a Orionis) and Bigel 
(/i Orionis), the first a variable star of a rich orange hue 
and the second of a blue- white radiance. 

Orion rejoices in the possession of about one hundred 



160 Astroxomy: New and Old. 

and fifteen stars visible to the unaided eye, of which 
four are of the second magnitude. 

Orion is also familiarly known as the " Yard and Ell." 

Taurus (The Bull). — This Constellation contains the 
tAvo remarkable clusters of the Pleiades and Hyades. 

The Hyades are shaped like the letter V, with the 
brilliant rose-red Aldebaran (a Tauri) in the upper left- 
hand corner. 

The Pleiades are also known as the Seven Stars. 
although only six are discernible to ordinary vision. 

The Pleiades make one of the most striking objects 
in the heavens, and have been celebrated in every 
age and country. 

This cluster is called by sailors the Hen-coop. 

A line from the Pleiades through Aldebaran and con- 
tinued south will almost pass through the middle of 
Orion. 

Gemini (The Twins J. — A short distance north by west 
from the Bee-hive are seen two stars of the first magni- 
tude, of almost equal brilliancy, Castor (a) and Pollux 
(/>), which give its name to this Constellation. 

Castor is white, with a tinge of green, and Pollux 
of a deep yellow color. 

Castor is a double star, and regarded by some obser- 
vers as a shade brighter than his twin brother, Pollux. 

Pollux is considered by many as only of the second 
magnitude. Others, however, look upon Pollux as the 
brighter of the Twins, and rate it of the first and Castor 
of the second magnitude. 

Pegasus. — This Constellation is a very conspicuous 
one from the fact that three of its own stars of the sec- 
ond magnitude, and one second-magnitude star of a 
neighboring group, form the figure of a large quadrilat- 
eral, called the Great Square of Pegasus. 



The Starry Heavens. 161 

About 9 o'clock p.m., on November 1 ; 11 o'clock 
P.M., October 1, and 1 o'clock on the morning of Sep- 
tember 1, the Great Square of Pegasus is almost directly 
overhead. 

Pegasus, Andromeda, and Perseus form a group of 
seven stars resembling very much the Great Bear. 

Three of the seven belong to Pegasus, three to 
Andromeda, and one to Perseus. 

This latter is Algol, a celebrated variable star. 

The star in the north-east corner of the square, 
together with the two stars in the handle in line with 
it, form Andromeda. 

In 1885 a great deal of attention was drawn to 
Andromeda, from the fact that what was supposed 
to be a new star suddenly shone out in line with its 
famous nebula. The phenomenon ceased after a few 
months. 

From the middle star in Andromeda two faint stars 
run north in a line with it. Close to the north end of 
this line is the great nebula of Andromeda, the oldest 
known to Astronomy. 

Piscis Australis (Southern Fish). — Looking along the 
meridian from the Great Square of Pegasus toward the 
south, there is seen almost on the horizon the beautiful 
first-magnitude star Fomalhaut, in the Constellation 
Piscis Australis. 

In the region between Pegasus and Fomalhaut lie 
the Constellations Aquarius (the Water-bearer), Capri- 
cornus (the Goat), Sagittarius (the Archer), Pisces 
(the Fishes), and Cetus (the Whale). 

These groups, with the exception of Cetus, have no 
remarkable stars. 

Cetus, which is south-east from Pegasus, contains 
the famous variable star Mira, that passes in eleven 



162 Astbonomy: New and Old. 

months from the second magnitude to the tenth, and 
back again to the second. 

Canis Major (The Great Dog). — This Constellation 
contains the very gem of the skies, the peerless Sirius. 
This glorious star is intensely white, with a suspicion 
of lilac ; is remarkably scintillating, and flashes like 
a diamond. 

Sirius is also called the Dog- Star ; it is south-east 
of Orion. 

Canis Minor (The Lesser Bog). — In this Constellation 
glitters Procyon, the little Dog-Star, a magnificent 
golden-yellow brilliant of the first magnitude. 

Sirius, Procyon, and Betel geuse form an equilateral 
triangle. 

Aries (the Earn), just east of the Pleiades; Mino- 
ceros (the Unicorn), south of Procyon; Hydra (the 
Water-Snake), south of the Sickle, have no remarkable 
stars. 

Lepns (the Hare), and Columba (the Dove), south 
of Rigel, have no conspicuous stars. 

Lyra (The Harp). — This Constellation possesses the 
brilliant bluish-white star Vega, one of the brightest 
first-magnitude stars in the northern heavens. 

A line from Arcturas through the brightest star 
of Corona, and continued easterly, will touch Vega. 

Hercules. — This Constellation lies between Yega and 
Corona, and contains about one hundred and fifty-five 
stars visible to the unassisted eye, two of which are of 
the second magnitude. 

The sun's journey through space is directed toward 
a point in Hercules. 

Draco (The Dragon). — The Dragon lies just north 
of Hercules. 

Coma Berenices (Berenice^ Hair J. — This is a beauti- 



The Starry Heavens. 163 

ful and striking cluster of small stars, lying close 
together between the Sickle and Arcturus. 

Virgo ( The Virgin). — The pure white first-magnitude 
star Spica graces this Constellation. 

Arcturus, Spica, and Denebola form an equilateral 
triangle. 

The Balance (Libra), the Crow (Corvus), and the Cup 
(Crater) are Constellations in the neighborhood of 
Spica. 

The Hunting Dogs (Canes Venatici), near Coma 
Berenices j the Little Lion (Leo Minor), north of the 
Sickle ; and the Serpent and Ophiuchus, just south of 
Corona, have no conspicuous stars. 

Cygnus (The Swan). — Alpha (a Cygni) of this Con- 
stellation is a first- magnitude star. Four other bright 
stars of Cygnus, together with Alpha, form a most 
perfect cross, lying along the Milky Way, and called 
the Northern Cross. It is a beautiful and striking 
group. 

Aquila (The Eagle). — The Eagle has a first-magnitude 
star, Altair. 

Altair, with Yega and a Cygni, forms an isosceles 
triangle. 

Auriga (The Charioteer). — This Constellation pos- 
sesses one of the greatest brilliants in the northern 
heavens, Capella. 

Auriga is a short distance east of Perseus and north 
of the Pleiades. 

Near Capella are three little stars forming a tiny 
triangle, and called the Hcecli, or Kids. 

Delphinus (The Dolphin). — This Constellation is 
northeast of the Eagle, and has a cluster of four stars 
in the form of a lozenge, called Job's Cofiin. 

Scorpio (The Scorpion). — At 8 o'clock p.m., on July 1, 



164 Astronomy: New and Old. 



Scorpio is seen just rising in the south-east. It is easily 
discernible by the fiery brick-red color of its first-mag- 
nitude star, Antares. This Constellation has a long, 
winding trail of stars, and bears a striking resemblance 
to a huge scorpion. 

The Fox (Vulpecula) and the Arrow (Sagitta) lie 
between the Swan and the Eagle. 

Cepheus, the Giraffe, and the Lynx are found 
between Cassiopeia and Polaris. 

The Triangles (Triangular), between Aries and An- 
dromeda ; the Fly (Musica), between Aries and Perseus; 
the Camelopard (Camelopardalus), north of Auriga; 
the Sextant (Sextans), south of Regulus ; the Owl 
(Nocta), near Virgo; Taurus Poniatowskii (the Polish 
Ball), near the Eagle; Scutum Sobieski (Sobieski's 
Shield), also near the Eagle; the Lizard (Lacerta), near 
the Swan, have no remarkable brilliants. 

Southern Constellations. — The list of Southern Con- 
stellations embraces the Southern Cross, the Centaur 
(Centaurus), the Wolf (Lupus), the Altar, the Southern 
Triangle, the Ship Argo (Argo Navis), the Flying Fish, 
Doradus, the Reticule, Eridanus (the River Po), the 
Pho'iiix, the Toucan, the Crane, the Indian, the Pea- 
cock, and the Southern Crown (Corona Australis). 

The Southern Cross and the Ship Argo are magnifi- 
cent groups indeed, and give to that region of the sky 
a splendor surpassed only by that of the huge hexagon 
at whose angles blaze Sirius, Rigel, Capella, Procyon, 
Aldebaran, and the Twins, with the ruddy glow of 
Betelgeuse marking the centre. 

Zodiacal Constellations. — There are twelve Constella- 
tions lying along the great highway of the sun, in each 
of which he passes a month. 

These are Aries (the Ram), Taurus (the Bull), 



The Starry Heavens. 165 

Gemini (the Twins), Cancer (the Crab), Leo (the Lion), 
Virgo (the Virgin), Libra (the Balance), Scorpio (the 
Scorpion), Sagittarius (the Archer), Capricornns (the 
Goat), Aquarius (the Water-Bearer), and Pisces (the 

Fishes). 

MYTHOLOGICAL HISTORY. 

Cassiopeia — Cassiopeia was the wife of Cepheus, the 
king of Ethiopia. She was of wondrous beauty. She 
boasted that her comeliness surpassed that of either 
Juno or the Nereides, or sea nymphs. This boast 
reaching the ears of the nymphs, filled them with 
resentment. These Nereides were great favorites with 
Neptune, the god of the sea. Neptune, to placate his 
favorite nymphs, sent a fearful sea-monster to ravage 
the country of Cassiopeia. Neptune refused to draw off 
the creature unless Andromeda, the lovely daughter of 
Cassiopeia, would be chained to a rock near the beach 
and exposed to the fury of the awful reptile. Perseus, 
returning from the conquest of the Gorgons, redeemed 
the beauteous maid by destroying the monster. 

Andromeda.— Andromeda was the daughter of Cepheus 
and Cassiopeia. On account of the resentment of the 
sea nymphs, provoked by Cassiopeia's boast, Neptune 
sent a gigantic monster to ravage the whole of Ethiopia. 
An oracle being consulted, it was declared that nothing 
could appease the anger of Neptune but the .sacrifice of 
Andromeda. She was, accordingly, chained to a rock 
on the Syrian coast. Perseus, arriving opportunely, 
through means of the Medusa's head which he was bear- 
ing home in triumph, turned the destroying monster 
into stone just as he was in the act of devouring the 
forlorn damsel. Perseus and Andromeda were married. 

Perseus. — Perseus was one of the most charming 



166 Astroxomy: New axd Old. 

characters of ancient fable. He was indeed an ideal 
hero. He was the son of Jupiter and Danse. Soon 
after his birth the child and his mother were cast into 
the sea. They were borne a great way by the winds 
and currents, and finally rescued by some fishermen off 
one of the islands of the Cyclades, and entrusted by the 
king of the island to the care of the priests of Minerva's 
temple. Perseus, by the courage- and nobility of his 
behavior, grew to be a great favorite with the gods. 
On a great feast-day of the king, his benefactor, Perseus 
promised to give him Medusa's head. The three Gor- 
dons were Medusa, Stheno, and Euryale. Medusa was 
the only one of the sisters subject to death. Their 
1 todies grew inseparably together, and were covered 
with impenetrable scales. These Gorgons had yellow 
wings and brazen hands, and their heads were 
wreathed round with numberless serpents. They were 
instantly turned to stone on whomsoever the eyes of 
the sisters were fixed. Pinto, the god of the Inferno, 
lent to Perseus a helmet having the virtue of rendering 
him invisible. Minerva, goddess of wisdom, gave him 
a polished buckler, and Mercury winged sandals. Con- 
dncted by Minerva, the hero came upon the sisters 
while they slept. He cut off Medusa's head with a 
single blow, and the immortal sisters, aroused by the 
noise, vainly pursued him, Pluto's helmet rendering him 
invisible. Perseus, Andromeda, and Cassiopeia were 
changed after death into Constellations. 

Orion. — According to one account, Orion was the son 
of Xeptune and a great huntress named Euryale. The 
hero took naturally to the chase, and became a hunter 
of high renown. Having vainglorionsly boasted that 
he could slay any animal of the forest, a scorpion sprung 
up out of the earth and stung him to death, in punish- 



The Stare y Heavexs. 167 

merit of liis vanity. Diana placed him among the stars, 
directly facing the celestial scorpion. In stature and 
strength Orion was said to surpass all mankind. The 
giant was skilled in the working of iron, and walled in 
the coast of Sicily against the encroachments of the sea. 

Gemini. — The Twins were the brothers Castor and 
Pollux, sons of Jupiter. They were among the heroes 
of golden-fleece fame, having joined the Argonautic 
expedition to Colchis, and performed prodigies of valor. 
Their supreme bravery is greatly extolled in fable; "one 
fought on foot, one curbed the fiery steed." In Grecian 
temples the brothers are represented riding side by side 
on white horses, armed with spears and their heads 
crowned with a glittering star. The Twins were looked 
upon as the friends of navigation, having driven all 
pirates from the Hellespont. 

Canis Major — The Great Dog very probably re- 
ceived its name from Egypt. The Nile was noticed 
to commence its periodic overflow just as Sirius, or 
the Dog-Star, of this Constellation rose in the east 
with the dawning of the day. Siris was the Egyptian 
name of the Nile, and hence Sirius, the great brilliant 
in Canis Major. Like a faithful dog, it gave warning of 
the approaching flood. 

It is also very likely that Canis Minor was likewise 
an Egyptian invention. It precedes the rising of Canis 
Major, and is a faithful sentinel of its approach. 

Ursa Major. — The Great Bear is understood to be 
Callisto, the daughter of a king of Arcadia. Jupiter 
became enthralled with Callisto's beauty, and Juno, in 
a fit of jealousy, changed her mortal rival into a bear. 
Jupiter placed Callisto among the Constellations. 

Coma Berenices — Berenice was the beautiful wife of 
Evergetes, one of Egypt's kings. She was fondly 



108 Astronomy: New axd Old. 

attached to her husband, and vowed to sacrifice her 
locks to Venus on the safe return of Evergetes 
from an expedition he had taken against the Assy- 
rians. Evergetes having reached home unharmed, 
in fulfilment of her vow Berenice placed her shorn hair 
in the temple of the goddess of beauty. The locks were 
stolen from their place in the temple, and Conon, the 
king's astronomer, haying discovered them in the sky, 
declared that Jupiter had placed them among the 
stars. 

Bootes. — According to the Greeks, Bootes, also called 
Areas, was the sou of Jupiter and Callisto. Bootes 
became a hunter of great repute. Callisto, the hero's 
mother, was transformed by the jealous Juno into a bear. 
Bootes one day in the chase roused a bear, which was 
his own mother, but unknown to him. Just as he was 
about to slay her Jupiter snatched both up to the sky, 
where he placed them among tin* stars. Bootes is some- 
times called Arctophylax, from the Greek of bear- 
keeper. 

Corona Borealis. — According to the historian Plu- 
tarch, this crown of seven stars (only six in the asterism) 
was given by Bacchus to his spouse Ariadne, which after 
her death was placed among the Constellations. 
Ariadne, the daughter of the second king of Crete, was 
very unhappy in her first marriage to Theseus, a hero 
greatly celebrated in Grecian story. Theseus was con- 
fined by the king's orders in the famous labyrinth of 
Crete, to be devoured by the awful Minotaur (half-man 
and half-bull). Theseus slew the monster, and escaped 
from the labyrinth through means of a clue furnished by 
Ariadne. Theseus and Ariadne were married. Ariadne 
was, however, soon forsaken by the faithless hero. Her 
second marriage was a very happy one. 



The Starry Heave xs. 169 

Lyra ( The Harp). — This is said to be tlie immortal 
harp of Orpheus, one of the Argonauts, and the father 
of song. It was given to him by Apollo. Orpheus 
played on this harp with so great a charm that his 
strains affected alike wild beasts, rivers, and mountains. 
The hero married Eurydice, one of the nymphs who 
used to listen to his songs. Eurydice, fleeing from the 
persecutions of an admirer, was bitten by a serpent and 
died of the venomous wound. Orpheus was inconsol- 
able, and resolved to recover her at every hazard. With 
his celestial lyre he entered hell, and awakened such 
melody that even the furies relented. Pluto and Proser- 
pine, the king and queen of the infernal regions, were 
moved to pity, and consented to let Eurydice go on con- 
dition that Orpheus would not look back while within 
the confines of their realm. The hero had almost 
reached the limits of Hades when he turned to catch a 
sight of the long-lost Eurydice. He saw her, but she 
immediately disappeared from his sight for ever. He 
was expelled from hell and denied all further admission. 
Orpheus was later put to death by Thracian women for 
misanthropy induced by his loss, his head cast into the 
Hebrus ; and as it was borne down the current to the 
iEgean Sea, the constant lips still continued to utter 
the name of Eurydice. 



MONTHLY ASPECT OP THE HEAVENS. 

The earth makes a complete circuit of the heavens in 
twelve months. In one month it journeys eastwardly 
30°, or the celestial vault turns every month apparently 
westward 30°. 

Owing to this apparent westward motion of the 



170 Astronomy: Xew axd Old. 

heavens, the Stars and Constellations rise two hours 
earlier each successive month. 

The Stars and Constellations that rise at midnight 
on January 1 will rise at 10 o'clock p.m. on February 
1, and 8 o'clock p.m. on March 1. 

The Stars rise two hours earlier every month , and in 
twelve months will gain twenty-four hours, or return to 
the place from whence we reckon. 

The following are the positions of the principal Stars 
and Constellations on or about the first day of each 
month of the year : 

January (8 p.m.) — In the East, the Manger just ris- 
ing; Procyon (Canis Minor) near the horizon ; the Twins 
higher up; Taurus (Aldebaran and the Pleiades) on the 
meridian. In the North, Cassiopeia and Perseus above 
the pole, and Ursa Major (Dipper) below and eastward. 
In the South-east, Orion. In the South-west, Fomal- 
haut (Piscis Australis). Perseus, Capella (Auriga), 
Andromeda, and Great Square near zenith. In the 
West, Job's Coffin (Delphinus), Altair (Eagle), and Vega 
(Lyra). 

February (8 p.m.) — In the East, the Sickle and Eegu- 
lus (Leo) just appearing. In the North, the Dipper lies 
east of the pole, and Cassiopeia west. The Twins are 
midway up the sky; Aldebaran (Taurus) overhead. In 
the South-east, Proeyon (Canis Minor) ; Sirius (Canis 
Major) ; Orion. North of the Zodiac, Capella (Auriga) 
east of the meridian ; Andromeda west of it. In the 
West, the Great Square of Pegasus sinking to the 
horizon. 

March (8 p.m.) — In the East, Berenice's Hair just 
appearing. In the North, the Dipper still east, and 
Cassiopeia west, of the pole. On the Sun's path, the 
Sickle (Lion), Castor and Pollux (Twins), and Aide- 



The Starry Heavens. 171 

baran (Taurus), west of the meridian. In the South-east, 
Procyon (Canis Minor) and Sirius (Canis Major). In 
the South, Orion. Capella (Auriga) in the zenith, and 
Andromeda west of the meridian. The Great Square 
just setting. 

April (8 p.m.) — In the East, Arcturus (Bootes) 
rising. In the North, the Dipper above and east of the 
pole, and Cassiopeia correspondingly west. On the 
Zodiacal road, Spica (Virgo) just appearing, Eegulus 
and the Sickle half-way up, the Bee-hive on the meri- 
dian, the great brilliants, Aldebaran (Taurus), Castor 
and Pollux (Gemini), going down in the west. In the 
North-east, Coma Berenices, Capella (Auriga), and 
Andromeda. 

May (8 p.m.) — In the East, the splendid Yega 
(a Lyrse) just appearing. On the Zodiacal road, Spica 
(Virgo) midway up, Eegulus and the Sickle (Leo) on the 
meridian, Castor and Pollux (Gemini), and Taurus 
(Aldebaran and the Pleiades) setting. In the North, 
the Dipper (Ursa Major) above the pole, and Cassiopeia 
below. In the South-east, the glowing fires of Sirius 
and Procyon ; Orion sinking in the west. North of the 
sun's path and east of the meridian shine the Northern 
Crown, Arcturus, and Coma Berenices. In the North- 
west, the lustrous Capella. 

June (8 p.m.) — In the East, the Swan and Eagle, 
with their first-magnitude gems, Alpha Cygni and Al- 
tair, are just appearing ; Vega (Lyra) and the Polish 
Bull have already arisen. On the Zodiac, the red 
Antares (Scorpio) just appearing; Spica on the meri- 
dian; Eegulus past; Taurus and Gemini toward the 
west. In the North, the Dipper above the pole, and 
Cassiopeia beneath. North of the Zodiacal highway, 
the Northern Crown and fiery Arcturus. In the North- 



172 Astronomy: New axd Old. 

west, Capella. Id the South-west, Procyon going 
down. 

July (8 p.m.)— In tlie East, Job's Coffin (Delphinus), 
Alpha of the Swan, and Vega. On the sun's path, 
Antares (Scorpio), Spica, and Sickle, with Regulus. 
The Northern Crown is almost in the zenith, and Arc- 
turns west of it. In the North, the Dipper is west, and 
Cassiopeia east, of the pole. 

August (8 p.m.) — In the East, Andromeda and 
Pegasus. In the North, Charles' Wain west of the 
pole, and Cassiopeia east. On the Zodiac, Antares 
(Scorpio) has passed the meridian, and Spica (Virgo) 
is sinking in the west. North of the Zodiacal road, 
Vega is just east of the meridian, and still further east 
are Job's Coffin and the Swan (Northern Cross). In the 
West, the Northern Crown and Arcturus (Bootes). In 
the North-west, Berenice's Hair. 

September (8 p.m.) — In the East, Perseus, Andro- 
meda, and Pegasus. In the North, the Plough (Ursa 
Major) below and to the west, and Cassiopeia above and 
east, of the pole. On the Zodiac, Antares (Scorpio) 
south-west; Spica (Virgo) sinking in the west. North 
of the Zodiacal road shines Vega (Lyra) on the meri- 
dian, to the east of which lie Job's Coffin, the Northern 
(ross (Swan), and Altair (Aquila); while west of the 
meridian is Taurus Poniatowskii (Polish Bull) ; and 
north of it, Corona, Arcturus, and Berenice's Hair. 

October (8 p.m.) — In the East, Andromeda half-way 
up the sky. On the Zodiacal road, Antares (Scorpio) 
is far down in the south-west. In the North, the Dipper 
is below and west, and Cassiopeia above and east, of the 
pole. North of the sun's path, Altair (Eagle) and Alpha 
Cygni (Swan) are on the meridian; Job's Coffin (Delphi- 
nus) east of it ; Vega is west of the meridian. In the 



The Starry Heavexs. 173 

North-west, the Northern Crown and Arcturus ; Bere- 
nice's Hair is sinking in the west. 

November (8 p.m.) — In the East, Capella (Auriga) 
is just appearing. On the Zodiac, Aldebaran (Taurus) 
is just rising. North of the sun's path, Andromeda and 
Great Square east of the meridian ; the Northern Cross 
(Swan), Job's Coffin (Dolphin), Altair (Aquila), and 
Yega (Lyra) west of the meridian. In the North, the 
Dipper is below, and Cassiopeia above, the pole. In the 
North-west, the Northern Crown. 

December (8 p.m.) — In the East, the great Orion is 
just appearing. On the sun's path the Twins are just 
appearing ; Aldebaran half-way up the heavens. In the 
North, the Great Bear is below, Cassiopeia above, and 
Perseus above and to the east of, the pole. In the 
North-east, Capella (Auriga). In the South, Fomalhaut 
(Piscis Australis) is west of the meridian. North of the 
Zodiacal highway, the Great Square is west of the 
meridian ; and still further west sparkle the Northern 
Cross (Cygnus), Job's Coffin, the luminous Vega 
(Harp), and the white Altair (Eagle). 



II. — THE STARS. 

To the unassisted eye the radical distinction between 
a Star and a planet is, the apparent immobility of the 
one relatively to the rest of the heavens, and the wan- 
dering of the other. 

Another difference is, that the Stars scintillate or 
sparkle more than the planets. 

There appears to be no very clear explanation of this 
nickering of the Stars. It is, however, most probable 
that the cause is j>artially atmospheric, and partially 



174 Astronomy: New axd Old. 

due to the interference of light-rays coming from such 
vast distances. 

When near the zenith the Star sparkles much less 
than when near the horizon, and the degree of scintilla- 
tion is governed in a great measure by the character 
of the weather. 

On certain nights none of the Stars seem to flicker, 
and on others again all the bright ones are noticed to 
scintillate strongly. 

Among the planets, Mercury, Venus, and Mars have 
a perceptible sparkle ; but Jupiter and Saturn scarcely 
ever flicker, and when they do it is of the feeblest 
character. 

If the atmosphere occasions this tremulousness, ought 
it not to afl'ect the planet equally with the Star f The 
planets have all a finite apparent size, while the Stars 
are the merest points. In the smallest telescopes the 
planets all have discs; in the most powerful optical 
instruments the Star is still a poiut. 

This is offered as a reason for the greater sparkle of 
the Star. Also, the light of the planets is faint compared 
with that of the Stars, for the latter are glowing suns. 

Again, on the summits of lofty mountains, where 
the atmosphere is much less restless than at lower 
altitudes, this tremulousness is very much diminished. 

This scintillation of the Stars is usually accompanied 
with a flashing of prismatic colors. 

Magnitudes. — The Stars have been divided into 
classes or magnitudes, according to their relative 
brightness. The lower the number that expresses 
a Star's magnitude, the brighter is the Star. 

All Stars visible to the unaided eye are comprised 
in the first six magnitudes. Stars of the seventh and 
lower magnitudes are telescopic. 



The Start? y He ave xs. 175 

The telescopic Stars are divided down to the twen- 
tieth magnitude, and even lower. 

Stars of the same magnitude are not of equal bright- 
ness 5 to class the Stars precisely according to their 
brightness would require a magnitude for every Star 
in the sky, as no two Stars are absolutely of the same 
lustre. 

It is, too, a very difficult matter to define the limits 
of the magnitudes, or where one ends and the other 
begins. It is, therefore, understood that this division 
of the Stars is conventional and arbitrary. 

Number of the Stars. — The number of Stars in both 
hemispheres visible to the unaided eye has been placed 
between five and six thousand. The number of Stars 
embraced in the different magnitudes increases rapidly 
as the lustre decreases. 

There are about 20 Stars of the first magnitude, 65 
of the second, 200 of the third, 500 of the fourth, 1,200 
of the fifth, and 4,000 of the sixth. 

Argelander, of Bonn, a great authority on Star cata- 
loguing, has given a chart of 314,926 Stars of the 
northern sky between the first and tenth magnitude. 

The British Association for the Advancement of Sci- 
ence gives a catalogue of 8,377 of the brightest Stars 
in both hemispheres. 

The Stars visible to the telescope have been variously 
estimated from forty to one hundred millions. 

Stellar Photometry. — There are many forms of the 
Photometer. A Photometer is an instrument for meas- 
uring the intensity of light. It may consist of a ver- 
tical screen of thin paper, a few inches from which is 
placed a cylindrical stick. 

When two lights are to be compared, they are so 
placed behind the stick that each casts a separate 



176 Astronomy: New axd Old. 

shadow of the stick upon the paper screen. The lights 
are moved to or from the stick until their shadows on 
the screen appear equally obscure. 

The squares of the distances of the lights from their 
shadows give the comparative intensities of the lights. 

Instead of the stick a second screen parallel to the 
first is sometimes employed. This second screen, which 
is much thicker than the first, has an aperture cut in 
its centre. 

When two lights apart from each other are placed 
behind the second screen they will cast separate 
illuminations through the aperture upon the first 
screen. 

The experimenter changes the relative distances 
of the lights from the second screen until the illumina- 
tions on the first screen appear of equal lustre. The 
relative intensities will be as the squares of the dis- 
tances of the lights from the first screen. 

Alpha Centauri is computed to emit four, Yega about 
forty, Sirius seventy-two, and Arcturus two hundred 
times as much light as our own sun. 

Professor Pickering has cultivated stellar photometry 
with such assiduity and success that it has assumed 
the rank of a separate branch of astronomic research. 
He has constructed a photometric catalogue of 4,260 
stars, from ninety thousand observations of light- 
intensity. 

According to the photometric measures of stellar 
brightness, it is estimated that the lustre of the Stars 
increases in geometrical progression, as the number of 
their magnitude decreases in arithmetical progression. 

The Stars of one magnitude are reputed two and one- 
half times brighter than those of the magnitude next 
below it. 



The Starry Heavens. Ill 

If the lustre of an average sixth-magnitude Star 
be taken as unity , that of the fifth-magnitude Stars will 
be 2£ ; of the fourth, 6£ ; of the third, 15| $ of the second, 
39^ ; and of the first about 98. 

Sirius, however, is computed to be five hundred 
times as bright as an average sixth-magnitude Star. 

It is also calculated that all the Stars of the first 
eight magnitudes give one hundred and forty-three 
times as much light as the well-known Yega in the 
Harp, and that all the Stars individually invisible to 
the unaided eye shed more light upon us than do all 
the visible or lucid Stars. 
First Magnitude Stars — 

"0 majestic Night ! 
Nature's great ancestor ! Day's elder born, 

And fated to survive the transient sun ! 
By mortals and immortals seen with awe ! 
A starry crown thy raven brow adorns, 
An azure zone thy waist ; clouds, in heaven's loom 
Wrought, through varieties of shape and shade, 
In ample folds of drapery divine, 

Thy flowing mantle form ; and heaven throughout 
Voluminously pour thy pompous train." — Young. 

The best authorities seem to rate the Stars of the first 
magnitude at twenty. Some limit the number to seven- 
teen, and others to fifteen. 

Sirius is the brightest Star in the sky, and shines 
in the constellation of the Great Dog. It is south- 
east of Orion, and not far from that great constella- 
tion. 

The spectroscope demonstrates that the Stars shine 
by their own light, and are glowing suns. 

Sirius is not only the brightest of Stars, but is a 
kingly sun. Sirius is about a million times as far from 



178 Astronomy: New and Old. 

us as our own sun, and is calculated to exceed it more 
than a thousand times in volume, and about twenty 
times in mass. 

Sirius is intensely white, with a delicate lilac tinge, 
and owes its brilliancy to incandescent hydrogen. Al- 
though, owing to its awful distance, Sirius appears to 
tin- unaided eye as fixed, still, according to the testi- 
mony of spectrum analysis, it is sweeping along through 
space at the rate of a thousand miles a minute. 

Eta Argus is the second Star in the order of bright- 
ness. Its home is in the southern sky, being invisible in 
our latitude. It is a singular Star, and varies very much 
in brightness. 

Canopus holds the third place among first-magnitude 
Stars. Being far down toward the south celestial pole, 
it is never visible north of the thirty-seventh degree of 
north latitude. 

Canopns is immeasurably farther away than Sirius. 
Canopus is certainly a much grander sun than Sirius, 
and, as far as present appearances go, it is the king of 
suns. It is a brilliant white Star. 

The next great brilliant is Alpha Centauri. It is a 
southern star, and invisible to us. It is the nearest of 
the fixed Stars whose distances have been computed, 
and is over two hundred thousand times more distant 
than the sun. 

Arcturus comes next in order, and in northern lati- 
tudes is second only to Sirius. It can be easily dis- 
cerned, its place being in the neighborhood of the well- 
known Northern Crown. Arcturus, when near the 
horizon, is of a deep red color, but when higher up is of 
a deep yellow. 

Arcturus is beyond sixteen hundred thousand times 
more distant than the sun, and although it journeys 



The Starry Heavens. 170 

through space at a rate of about fifty-four miles a 
second, still, owing to its enormous distance, it appears 
to travel only the eighth part of the moon's diameter in 
a century. 

Capella, Vega, Procyon, Betelgeuse, and Eigel suc- 
ceed Arcturus in the order named. 

Betelgeuse (a Orionis) and Eigel (/3 Orionis) are the 
two bright Stars of Orion. The lower one, a spark- 
ling white Star with a bluish tinge, is Eigel. Betel- 
geuse is a variable Star, of a rich deep orange hue. 

Capella has a creamy or pearly lustre, and can be 
found by prolonging the line joining the two stars of the 
quadrilateral of the Great Bear nearest the pole. 

Yega is a Star of brilliant whiteness, with a bluish 
tinge, and can be found at the corner of a triangle of 
which Arcturus and Polaris form the base. Yega owes 
its intense whiteness to glowing hydrogen. 

Procyon is of a golden-yellow color, and, with Sirius 
and Betelgeuse, forms an equilateral triangle. 

Achernar (a Eridani), Beta Centauri, Altair, Alpha 
Crucis, Aldebaran, Fomalhaut, Beta Crucis, Pollux, 
Antares, Eegulus, and Spica follow Eigel in the order 
of their lustre. 

Of these, Achernar, Beta Centauri, Alpha Crucis, 
and Beta Crucis belong to the southern sky, and are 
invisible to us. 

Fomalhaut, though a southern Star, can be seen in 
our latitude low down toward the southern horizon. It 
is a beautiful Star with a slight reddish tint. 

Aldebaran (Alpha Tauri) is of a rose-red color, and 
resembles the planet Mars. It is close to the well- 
known cluster of the Pleiades, or Hen-coop. 

Antares (a Scorpii) is of a fire-red color. In summer 
evenings a line from the Pole Star through the edge of 



180 Astronomy: New axd Old. 

the Northern Crown will strike, near the southern 
horizon, this brilliant red Star. 

When Vega is in the zenith, and Antares on the 
western horizon, Altair, a brilliant white star, will be 
found in a line midway between them. 

A line drawn from the pointer nearest Polaris to the 
Star in the cup of the Dipper, diagonally across, and 
prolonged, will pass through Spica. 

Spica is remarkable for its pure white light. 

Pollux, of the Twins, is of a deep yellow color. It is 
the southernmost Star of Gemini, and, with Procyon 
and 35etelgeu.se, forms a right-angled triangle, Procyon 
being at the right angle. 

Regulus is the beautiful white Star in the handle of 
the Sickle. 

Stellar Distances — To compute the distance to a fixed 
Star it is necessary to find its annual parallax. This 
annual parallax is the angle subtended by the radius 
of the earth's orbit at the Star's distance. It is the 
magnitude of the sun's mean distance as seen from the 
Star. 

When it is considered that the motion of the earth 
around the sun brings us, at one time, a whole diameter 
of its orbit (one hundred and eighty-five millions of 
miles) nearer to a particular part of the heavens than 
we were six months before, we should expect a change 
in the relative distances of the Stars, as seen from the 
two points ; that as we approached them they should 
seem to separate. 

But owing to the awful distances of the Stars no such 
change, except in a few rare instances, is noticed to 
occur, eveu by the most delicate and powerful optical 
contrivances. 

This journey of the earth from a point in its orbit to 



The Starry He avexs. 181 

the opposite one gives an apparent oscillation of 6° 
to the planet Saturn on each side of its mean posi- 
tion. 

The annual parallax of a Star, however, is only half 
its apparent displacement by a six months' journey of 
the earth. The distance of a body from the sun is 
almost inversely as the parallax. 

If the annual parallax of a Star amounted to one 
second of an arc, the Star's distance would be about 
206,000 times that of the sun. 

Bessel, of Konigsberg, published in March, 1840, the 
first parallax of a fixed Star. It was of the Star num- 
bered 61 in the constellation of the Swan. Bessel made 
the parallax 0".3483, corresponding to 600,000 radii 
of the earth's orbit. 

Peters, of Pulkowa, confirmed the result in 1842. 

Dr. Ball, of Dunsink, afterwards made the parallax 
0".47, and Professor Hall, of Washington, 0".48. 

Newcoinb and Holden make the parallax 0".51, cor- 
responding to 400,000 radii of the orbit of the earth, and 
this is probably very near the truth. 

Thomas Henderson found the parallax of Alpha Cen- 
tauri to be almost one second of an arc, and Alpha is 
regarded as the nearest fixed Star. 

Guillemin gives the following table of the eight most 
accurately measured Star distances, expressed in astro- 
nomical units, or radii of the earth's orbit : 

Radii of 
Earth's Orbit. 

e Ursa3 Majoris, 1,550,800 
Arcturus, - - 1,622,800 
Polaris, - - - 3,078,600 
Sirius, - 1,375,000 Capella, - - - 4,484,000 





Radii of 

Earth's Orbit. 


a Centauri, - 


211,330 


61 Cygni, - 


- - 550,920 


Vega, - - - 


- 1,330,700 



182 Astroxomy: Xew and Old. 



The astronomical units may be converted into miles 
by multiplying them by 92,500,000: 

Miles. 

a Oentauri, ----- 19,548,025,000,000 

OlCygni, 50,900,100,000,000 

Vega, 123,089,750,000,000 

Sirius, 127,187,500,000,000 

£ Ursa Majoris, - - - - 143,449,000,000,000 

Arcturus, 150,109,000,000,000' 

Polaris, 284,700,500,000,000 

Capella, 404,770,000,000,000 

Drs. Gill and Elkin have recently ascertained that 
the distance of Alpha Oentauri is one-third greater than 
Henderson had computed it. They found, too, the 
parallax of Sirius to be double what it had been previ- 
ously reputed, or reduced the Star's distance by one- 
half. 

It is seen from the Star distances obtained that the 
brightest ones are not always the nearest to us. 

Double Stars. — A great number of Stars, appearing- to 
the unaided eye as single luminous points, when viewed 
through the telescope are seen to be double, or even 
multiple. 

In some instances the proximity of the Stars to each 
other is attributable to the effect of perspective, the 
Stars lying in the same line of light, although they 
themselves are separated by an immeasurable dis- 
tance. 

In many other cases the Stars are about equally dis- 
tant from us, are comparatively close together, are phy- 
sically connected, and actually form pairs, or Star 
systems. 

The physically double, however, are the only ones 
that ordinarily receive the name of double Stars. 



The Starry Heavens. 183 

Cassini, in 1678, first drew attention to the subject of 
double Stars. 

Bode, in 1781, published a list of eighty double 
Stars. 

Burnham, of the Lick Observatory, who is probably 
the best living authority on this subject, has published 
a catalogue of 1,025 new double Stars, to which he 
later added 42 new ones observed on Mount Hamil- 
ton. 

These double and multiple Stars obey the laws of 
gravitation, and revolve about one another, or, rather, 
about a common centre, in regular periods, varying from 
18 to 1,625 years. 

The shortest computed period is that of 10.8 years, 
by Otto Struve, 1852, for the pair known as 6 (Delta) 
Equulei, and the longest that of 1624.8 years, by 
Doberck, 1877, for 8, (Zeta) Aquarii. 

The distance to 61 Cygni has been measured. This 
is a double Star, whose components are separated from 
one another by forty-five times the distance of the earth 
to the sun. 

Yet their distance from us is so prodigious that the 
breadth of the great gulf that divides them is invisible 
to the unassisted eye, and we require the aid of a power- 
ful telescope to part them. 

Variable Stars. — Stars whose brightness undergoes 
periodic variations are called variables. 

There are over two hundred known variable Stars, 
and a still larger number of suspected ones. Indeed, 
Mr. J. E. Gore gives a catalogue of seven hundred and 
thirty-six suspected variable Stars. 

Probably the most famous of all the variables, on 
account of the great range of its fluctuations, is the Star 
marked in the catalogues as Omicron Oeti. 



184 Astroxomy: Xew and Old. 

It was the first known periodical Star, its variability 
having been noticed by Fabricius, in 159G. 

Hevelius called it " Mira," or the Wonderful, which 
name it has since retained. 

Its mean period is about 331^ days. It changes from 
the second to the tenth magnitude, or it is a sun that 
shines at one time a thousand times more brilliantly 
than at another. 

During five months it is entirely invisible to the un- 
aided eye ; it then begins to appear again, increasing 
slowly in lustre for three months, until it shines as a star 
of the second magnitude. It holds this brilliancy for 
about two weeks and then begins to fade, and in three 
months again disappears. 

Its periodicity is subject to irregularities, however, 
and the Star does not reach its greatest lustre after 
every period. 

The cause of these changes of lustre in the variables 
is still unknown. Various working hypotheses are 
advanced to account for the phenomena, such as the 
revolution of a dark body around the luminous one ; 
the rotation of the body itself, its sides differing in 
luminosity, or one side being completely dark, or spots of 
great dimensions, similar to sun-spots, floating over the 
surfaces. 

It is thought that the fluctuations of Mira's light 
are jointly due to a dense absorbing atmosphere and great 
floating spots. Mira is regarded as a dying sun. It has 
a slightly reddish tint, and indeed the variables as a 
ride are of a ruddy hue. 

Another noted variable is Algol, or Beta Persei. 
During two days and a half Algol, or the Demon, as 
the Arabs call it, is a Star of the second magnitude ; it 
then begins to fade, and in about four and a half hours 



The Starry Heavens. 185 

it sinks to the fourth magnitude. It remains, however, 
only a few minutes so faint as this, and begins then to 
again brighten, until in about four and a half hours 
more it regains its first lustre. All these changes in 
Algol are plainly visible to the unassisted eye. 

The " eclipse" theory seems to fit the case of Algol 
better than that of any other variable. It is a white 
Star, and closely resembles Sirius. If its variability 
depended on an absorptive atmosphere, its individual 
rays would be attacked, or its color would vary with the 
change of lustre. 

Algol is constant in respect to color, always remain- 
ing white. Again, the time of its obscuration is but 
the merest fraction of the whole period, and this would 
favor the satellite or eclipse theory. 

Goodricke, in 1782, first proposed the hypothesis 
of a satellite revolving around Algol. It has held its 
ground since. 

This hypothesis requires that the satellite be an enor- 
mous body, its diameter 0.764 times that of the Star, 
and its period of revolution around the primary two 
days, twenty hours, and forty-nine minutes. 

Against this theory it must be urged that it is 
extremely hard to believe that an immense body is 
travelling around another, still more enormous, with 
only four thousand miles between their surfaces. 

Another wonderful variable is Eta Argus of the 
southern skies. This singular Star varies from the 
sixth magnitude to a brilliancy exceeding that of 
Canopus, and approaching closely to the lustre of even 
Sirius. No law can be found governing its periodicity 
or its changes of brightness. 

Temporary Stars — Stars that suddenly appear, and 
after shining more or less brilliantly for a short period, 



1BC Astronomy: New and Old. 

either disappear altogether or remain as very faint 
objects, are called temporary or new Stars. 

Tycho Brahe, in November, 1572, saw a very bright 
new star in Cassiopeia, which grew in lustre until it 
almost rivalled Venus, and after gradually fading during 
a period of seventeen months, finally disappeared from 
view. 

We have records of above twenty of these temporary 
or new Stars. 

On May 12, 18GG, a new Star of the second magni- 
tude appeared in the Northern Crown which certainly 
was not shining there four hours previously. It became 
invisible to the unaided eye nine days after its discovery, 
and it is now in the Star maps as a pale yellow Star of 
the tenth magnitude. Its sudden glow of lustre was 
due, according to the spectroscope, to incandescent 
hydrogen. 

Dr. Schmidt discovered, on the 24th of November, 
1870, a new Star of the third magnitude in the Swan, 
which is known under the name of Nova Cygni. Its 
spectrum was nearly similar in character to that of the 
new ('/') Star in Corona. 

It appears that Nova Cygni later changed into a 
planetary nebula. This extraordinary object now ap- 
pears as a telescopic Star of the fourteenth magnitude. 

What is the cause of the phenomena of temporary 
Stars % Rapid motion, electric and magnetic influences 
(Humboldt), the intervention of nebulous masses not 
self-luminous, and other improbable causes have been 
variously assigned. 

The most plausible theory, however, seems to be that 
this sudden glow of light in temporary stars is due to 
the eruption of incandescent hydrogen from the in- 
terior. 



The Starry Heavens. 187 

Stellar Spectra. — Spectrum analysis enables us to 
affirm the presence or absence of certain substances in 
any light-source whatever, so that we can say from the 
spectroscopic observation of a Star's light whether or 
not it contains hydrogen, iron, copper, or other ele- 
ment. 

Secchi, Huggins, and Miller were the principal 
founders of stellar spectroscopy. 

Secchi was the first to make a spectroscopic survey 
of the heavens. He examined more than 4,000 Stars, 
w r hich he classified, according to the character of their 
spectra, into four types. 

The first type is called the Sirian, and embraces all 
the bluish-white Stars resembling Sirius and Eegulus. 

This spectrum is continuous, and crossed by four 
broad and intense dark bands, due to great quantities 
of hydrogen present in the stellar atmosphere. A 
number of very faint metallic lines are also perceptible 
in this spectrum. 

The second type is called the Solar, and comprises 
all the yellow Stars resembling Aldebaran and our own 
sun. This spectrum bears a close analogy to that of 
the sun. 

The third type comprises the bright red Stars 
resembling Betelgeuse and Antares. This spectrum 
is of the " fluted" kind, and has the appearance of 
a luminous colonnade, and is due to the absorption 
of light by the vapors of compound substances in the 
stellar atmospheres. 

The fourth type is composed of telescopic red Stars. 
These Stars are of a very deep red and gleam like 
rubies; the peculiarity of the spectrum of this type 
appears to be due to the presence of carbon in their 
atmospheres. 



188 Astronomy: New axd Old. 

The spectrum of this type appears fluted, and Las 
three Large bright spaces divided by darker ones. The 
spectra of all the Stars prove thein to be suns, only 
differing from one another in the absorptive nature of 
their atmospheres. 

The hotter a Star is, very probably, the simpler is its 
spectrum; it is not proven, however, though often 
asserted, that the white Stars are young suns, and the 
red ones expiring suns. 

Their spectra give evidence that hydrogen, sodium, 
iron, and magnesium are present in almost all the Stars 
whose light has been analyzed; and the spectrum 
of Aldebaran, which has been most carefully studied, 
reveals the presence id that Star of hydrogen, sodium, 
magnesium, ealeium, iron, bismuth, tellurium, antimony, 
and mercury. 

Colored Stars. — The twinkling of the Stars is usually 
accompanied with a flashing of different colors; but 
apart from the ever-changing tints occasioned by scin- 
tillation, they have real and permanent hues. 

The greatest variety and contrast of color are found, 
however, among the double Stars. 

We find combinations of green and red, orange and 
blue, yellow and purple, gold and lilac, white and blue, 
white and green, in Star pairs. 

There are ash-colored, citron, fawn, mauve, puce, 
russet, and olive Stars. A decided green, blue, purple, 
or brown is never met in single Stars, and a cinnamon- 
colored Star is rare. 

It seems to be the testimony of the spectroscope that 
the colors of the Stars are due to the nature of their 
vaporous envelopes. 

lluggins, in 1804, laid down the principle that the 
colors of the Stars depend less on the intrinsic nature 



The Starry Heavens. 189 

of their light than on the elimination by their atmos- 
pheres of certain light-rays. 

Each Star really emits white light, but this light 
shines in one instance through bluish vapors, and comes 
out blue ; and in another through orange vapors, and 
comes out tinged with orange. 

The great majority of the lucid Stars are white or 
bluish-white; the yellow Stars come next in number, 
while only about two per cent, of the naked-eye Stars 
are ruddy. 

Taurus and Orion are remarkable for their vast gath- 
erings of white Stars, and Oetus and Pisces for their 
aggregations of yellow ones. 

The estimations that have been made of Star colors 
are very unsatisfactory, owing to the wide difference 
between observers in this respect. 

A very valuable catalogue of 650 red Stars was pub- 
lished by Birmingham in 1877, under the title of Red 
Star Catalogue. 

Errors may arise, principally from three sources, 
in observing Star colors. These sources are the atmos- 
phere, the telescope, and the eye itself. 

Stars less than 20° above the horizon should never 
be observed for color, and haze and fog should be 
avoided. 

The red appearance of the sun when low in the sky 
or enveloped in fog proves this. 

The best telescope to employ in the observation of 
Star colors is a silvered-glass reflector. 

If a refractor be used it should be of moderate 
aperture, as large lenses, however well corrected, 
are not perfectly achromatic, and throw a blue halo 
around the object. The silvered specula are practically 
free from this defect, 



190 Astronomy: New and Old. 

The eye-piece should also be as perfectly corrected 
for dispersion as possible, and the Star kept in the 
centre of the field of view, as the margin is never 
entirely achromatic 

Different eyes make different estimates of colors. 
We tind great discrepancies among observers, shown 
in the tints assigned to Stars. Thus, it is well known 
that Strove was partial to red tints, and Secchi to 
yellow ones. 

It frequently happens that persons with the greatest 
acuteness of vision have not a perfect color appre- 
ciation. 

Owing to nervousness, strain, or illness, the same 
eye does not always form the same estimate of a color. 
The first good look at a Star is the best, as continued 
scrutiny distresses the eye, and a different hue is per- 
haps then assigned. 

In examining Stars for color artificial light should be 
avoided by the observer as much as possible in order to 
form a proper estimate, as nearly all artificial light has 
a yellow tint, and biases the eye accordingly. 

Stellar Motion — To the unarmed eye the Stars appear 

to be relatively immovable. With the aid of very 

powerful and delicate optical instruments, however, 

a small apparent relative stellar displacement is dis- 

* cernible. 

Thus, for instance, the Star 61 Cygni moves appar- 
ently one-third of the moon's diameter across the sky in 
a hundred years. In the same interval Alpha Centauri 
suffers an apparent displacement of one-fifth, and the 
brilliant Arcturus one-eighth, of the lunar diameter. 

The distances of a few of the Stars have been meas- 
ured. When the stellar distance and apparent velocity 
are known, the real stellar velocity is easily computed. 



The Starry Heavexs. 191 



Guillemin gives the following table of the velocities 
of seven Stars : 

Miles a Second. Miles a Second. 

Arc turns, 51 Alpha Centauri, - - 13 

61 Cygni, - - - - 40 Vega, 13 

Capella, 30 Polaris, 1J 

Sirius, 11 

The spectroscope affords a method of computing the 
rate of velocity of the Stars in the line of sight, or in 
a direction to or from the earth. 

In the solar and stellar spectra every element has its 
characteristic lines, and these lines, moreover, have 
their permanent places. 

It has been noticed that the spectrum of a moving 
body shifts according to the direction of the motion. It 
is the spectrum as a whole that is driven hither or 
thither by the motion, the separate lines maintaining 
their relative distances. 

It is proved by experiment that if the light-source is 
moving toward us, the spectrum will be shifted toward 
the violet end ; and if away from us, it will be shifted 
toward the red end. 

By measuring the spectral displacement the rate of 
velocity can be computed. After a great deal of care 
and industry, physicists have been able to make a scale 
showing the relation between the rate of velocity and 
the amount of displacement. 

Professor Young was enabled, in 1876, to verify the 
correctness of the scale. 

From the observations of the motion of spots on the 
sun, it is estimated that his eastern edge moves toward 
us with an equatorial velocity of a mile and a quarter 
a second, and his western edge recedes at the same rate. 



192 Astronomy: New and Old. 

The spectroscope, in the hands of Professor Young, 
showed almost the same speed for the sun's axial 
rotation. 

Sirius, Betelgeuse, Rigel, and Regulus are retreating 
from us, and Arcturus, Pollux, Vega, and Deneb of the 
Swan arc approaching as. 

The rate of the sun's translation through space has 
been computed, and its direction indicated. 

The computed rate of the sun's speed is four miles 
a second, and, with some very slight discrepancies, Arge- 
lander, Struve, Miidler, Airy, Dunkin, and others make 
the direction north toward Hercules, and away from 
Argus in the south. 

A Star of the seventh magnitude, numbered 1,830 
in Groombridge's catalogue, has the greatest proper 
motion of any in the heavens, its speed being estimated 
at two hundred miles a second. 

Stellar Masses. — When the distance of a heavenly 
body and the apparent size of its disc are known quan- 
tities, its real dimensions or bulk can be easily deter- 
mined. 

The distances of some few of the Stars have been 
computed; but the most powerful telescope that has yet 
been made fails to give a single one of them an appreci- 
able disc. So that no measure has ever yet been made 
of a Star's size. The mass or weight, however, of some 
few of the Stars has been calculated. 

We know that there are numbers of double Stars, 
or of two stars revolving about one another and form- 
ing systems. 

When the apparent distance apart of the Stars com- 
posing one of these doubles has been determined, and 
the real distance from us of the pair is known, the true 
distance in miles that separates them can be calculated. 



Thus Starry Heavens. 193 

And when we know their real distance apart in miles, 
and the periodic time of their revolution, their united 
mass, in terms of that of the snu, can easily be calcu- 
lated. 

The components of Alpha Centanri revolve abont 
their common centre at a mean distance of 23^ radii of 
the earth's orbit in a period of about 77£ years. From 
this it is computed that their united mass is twice the 
sun's. 

The Star 61 Cygni weighs about one-third of the 
sun. 

Sirius and its companion circle about their common 
centre in about 49 years, and are sundered from each 
other by about 37 radii of the earth's orbit. 

According to Kepler's third law (the squares of the 
times of revolution of the planetary bodies are propor- 
tioned to the cubes of their mean distances from the 
sun), a body situated 37 times more distant from the 
sun than our earth would have a periodic time of about 
225 years. 

The combined mass of Sirius and its companion is to 
the sun's as the square of 225 is to the square of 49, or 
as 21 to 1. Masses are inversely proportioned to the 
square of the periodic time. 

HI. — STAR-CLUSTERS AND NEBULA. 

Star-Clusters. — We easily notice that the Stars are 
not uniformly spread out over the sky. Some portions 
of the celestial vault are much richer in stellar jewels 
than others. 

Frequently quite a large number of Stars are seen 
gathered together within a small area. These bunches 
of Stars are called clusters. 



194 Astronomy: New axd Old. 

It is very probable that the grouping together of 
these Stars is not clue to chance, but that the indi- 
viduals are physically connected and form great Star 
systems. 

The unaided eye easily distinguishes many of the 
coarser clusters or groups, such as the Pleiades, the 
Hyades, Berenice's Hair, and Praesepe, or the Manger. 

There are other clusters where the individual Stars 
cannot be discerned by the unarmed eye, and which 
appear without telescopic aid as Nebulae, or whitish 
cloud patches. 

Star-clusters are ordinarily of a rounded or globu- 
lar form, although sometimes, too, they are of an ir- 
regular shape. 

The clusters that look to the unaided eye as faint 
vaporous Stars maybe easily resolved by a good tele- 
scope into individual Stars. There are numbers of 
clusters visible only in the telescope. 

Nearly all Star-clusters appear in the telescope to 
have a remarkable condensation of light toward the 
centre. This is owing partly to perspective, and partly 
to a leal condensation of the Stars, due to the influence 
of the central forces of the stellar system. 

These little Stars composing the clusters, buried 
in space to such prodigious depths, and therefore 
appearing to us so pressed together, are in reality 
probably divided from each other as widely as our own 
sun is from its nearest neighbors among the Stars. In 
some of these minute clusters there are many thousands 
of Stars. 

One of the most beautiful and striking of the clusters 
is in the constellation Hercules, and when seen with 
a great telescope is truly a glorious object, being con- 
sidered by many observers unsurpassed, if, indeed, 



The Starry Heavens. 195 

rivalled in splendor by any other telescopic sight in the 
northern sky. 

The cluster is close to the Northern Crown, and 
in a line between the Crown and the brilliant Vega. 
The number of Stars in the cluster is reckoned at four- 
teen thousand. 

A cluster of great beauty is the southern one in 
Toucan, and is visible to the unaided eye. Its centre 
is of an orange-red color, and its border white. 

The clusters in the Centaur, Aquarius, and Perseus 
are visible to the unaided eye. 

The greatest number of the Star-clusters is found 
along the Milky Way. 

Nebulae. — All the little whitish clouds, or luminous 
vaporous masses, scattered over various portions of the 
sky, go under the generic name of Nebulae. 

These objects are easily discerned from the clouds 
floating in our atmosphere by their relative immobility 
in space. 

Some of the nebulae are Star-clusters, which small 
telescopes can readily divide up into individual Stars. 

Other nebulae are partly resolvable, by a great tele- 
scope, into Star points, and partly not resolvable. 

Another class of nebulae absolutely defy resolution 
by the most powerful instruments. 

Nebulae of Regular Forms. — There are nebulae of 
regular forms, such as the globular or spherical, the 
elliptical, the annular, and the spiral nebulae. 

Specimens of the annular nebulae are found in the 
Harp, Swan, Scorjrion, and Andromeda ; and of the spiral 
nebulae in the Hunting Dogs, Virgin, Lion, and Pegasus. 

Nebulae of Irregular Forms. — There are also the irreg- 
ular nebulae, of every conceivable form and entirely 
devoid of all symmetry. 



19G Astronomy: New axd Old. 

Of the irregular class specimens are found in So- 
bieski's Shield, Taurus, Doradus, Orion, and Argus. 
These nebulae have the most varied and fantastic 
shapes. 

What appears as a regular nebula in a small tele- 
scope is not uufrequently resolved into an irregular 
one by a more powerful instrument. 

Famous Nebulae. — The most famous of all the nebulae 
are the great ones in Andromeda and Orion. 

The only nebula known before the invention of the 
telescope was that in Andromeda. It was recognized 
by the Persian astronomer Abdurrahman a thousand 
years ago. 

The attention of observers was first, however, prac- 
tically drawn to it by Simon Marias in 1012. 

Huyghens may be said to have been the discoverer 
of the nebula in Orion in 1G5G, although it had been 
previously mentioned by Cysatus, in 1G18, as a term 
of comparison with a comet. 

There is a farther division of nebulae into the plane- 
tary nebulae and nebulous Stars. 

Planetary Nebula?. — A planetary nebula is one whose 
disc is circular and of uniform lustre. These nebulae 
have no condensation of light toward the centre, and 
resemble very much in appearance the discs of the outer 
planets seen in a telescope. 

There is a planetary nebula iu the Great Bear, and 
one in Andromeda. 

Nebulous Stars — A nebulous Star is a nebula of regu- 
lar form, in the interior of which appears one or more 
Stars symmetrically placed. 

If it be of the circular form, the Star is a\ the centre; 
if of the elliptical, two Stars are found situated at the 
curve's foci. 



The Starry Heavens. 197 

The Stars, in all cases, are quite distinct from the 
nebulae. 

There are double and multiple nebulae forming sys- 
tems. There are also variable nebula?, whose light 
undergoes frequent changes. 

A catalogue containing 6,251 nebulae has been 
recently published. 

The portions of the sky nearest the Milky Way are 
the poorest in nebula?, while the regions around the 
poles of the great galactic belt have the greatest nebular 
wealth. 

The constellation of the Virgin is richer in nebulae 
than any other region of the sky. 

Nebular Spectra. — The question as to whether all 
nebulae could be resolved into stellar points were the 
optical instrument sufficiently powerful, has been 
answered satisfactorily by spectrum analysis. 

The spectroscope has demonstrated that some of the 
nebulae are not Star-clusters, but self-luminous, diffused 
vaporous matter. 

On August 29, 1861, Dr. Huggins obtained the spec- 
trum of the planetary nebula in the Dragon. This 
spectrum consisted of three lines, one fairly bright, and 
the others exceedingly faint. 

It was the discontinuous spectrum of glowing gas, 
and radically differed from the stellar continuous 
spectra. 

The bright line was due to nitrogen, and one of the 
faint ones to hydrogen ; the substance of the third line 
is unknown. 

The spectra of clusters are continuous, are crossed 
by dark lines similar to that of the sun, and show the 
light to be that of a solid or fluid body with a vaporous 
envelope. 



108 Astroxomy: Sew axd Old. 



The spectrum of the gaseous nebula, on the contrary, 
is that of glowing gas. 

Dr. Huggins afterwards analyzed the light of as 
many as seventy nebulae, and found that one-third of the 
number had the characteristic spectra of incandescent 
gas. 

All the planetary, annular, and irregular nebulae 
belong to the gaseous kind. 

The great nebula in Andromeda, the spiral one in the 
Hunting Dogs, and, as a rule, all nebulae reducible to 
clusters, give a continuous or stellar spectrum. 

The Orion nebula has a discontinuous or gaseous 
spectrum. 

Immobility of Nebulae.— Some of the Stars are noticed 
to have slight apparent motions; the nebulae, however, 
seem absolutely immovable. 

The spectroscope shows most of the Stars that have 
been examined to have a motion in the line of sight, but 
no trace of displacement has ever been noticed in the 
nebulae, even by this delicate test. 

Not External Universes.— The old conceptions of La- 
place and William Herschel about nebulae being re- 
mote galaxies, or external universes, are now given 
up. 

The analysis of the Magellanic Clouds, or the two 
great nebulae near the Southern Pole, retired such 
notions entirely. 

In the greater cloud, or Nubecula Major, as it is 
called, are found mixed together indiscriminately Stars, 
clusters, regular and irregular nebulae, and nebulous 
streaks. 

It cannot be maintained, then, that nebulas are 
remote worlds of Stars, as some at least of them cer- 
tainly lie within the limits of the sidereal system ; and 



The Staff y He a vexs. 199 

it is more than probable that the Stars and nebulae are 
parts of a single scheme. 

The Milky Way.— The Milky Way, or Galaxy, is the 
luminous zone or long nebulous train which stretches 
across the sky from horizon to horizon. 

When completely traced this whitish band is found 
to extend, in the form of a great circle, around the 
whole celestial vault. 

The outlines of this Star cloud are irregular and 
broken along its whole course, and its glimmer very 
variable. 

The zone of cloudy light divides into two branches, 
one faint and interrupted, and the other comparatively 
bright and continuous. The branches, after remaining 
apart for a distance of 150°, again unite. 

The brightest part of the Milky Way in the northern 
hemisphere traverses the Eagle and the Swan; but the 
brightest portion of the whole galactic highway runs 
through the southern asterisms of Argus and the Altar. 

Where the Galaxy enters the Southern Cross, and 
where its breadth is narrowest, is seen the famous pear- 
shaped opening called by mariners the Coal Sack. 

The breadth of the Milky Way is four times greater 
in some places than in others. 

The Galactic Circle cuts the equinoctial or celestial 
equator at an angle of 63°, and at points close to the 
great Orion and Ophiuchus. 

The Milky Way owes its light to irradiation, or the 
united lustre of vast numbers of very faint Stars. 

The Milky Way can be resolved with a good tele- 
scope almost entirely into Stars. William Herschel 
estimated the number of Stars in the Galaxy at eighteen 
millions. 

The poles of the Galactic Circle are in Berenice's 



200 Astronomy : New axd Old. 

Hair and the Whale. It is estimated that the Stars 
are thirty times more numerous in the Galactic Circle 
than around its poles. 

Structure of the Universe. — The great Milky Way, 
embracing the sun and all the Stars we see, is the vis- 
ible or stellar universe. 

All observers agree that our sun is a member of this 
sidereal system, and we can never, consequently, have 
a proper idea of its true appearance. To form an abso- 
lutely true conception of the structure of this system 
of Stars we should be able to view it from the 
outside. 

Thomas Wright, of Durham, formed the first definite 
notion of its construction, and is the father of the 
"Grindstone Theory," or that the stellar system has the 
shape of a flattened millstone. 

Kant came next with his theory of " Island Uni- 
verses. 1 ' He regarded the nebulae as external universes, 
equal in size to the Milky Way. His hypothesis was 
taken from his imagination, and has no scientific value. 

William Hersehel, after the noble work of surveying 
the whole heavens with his great telescopes, proposed 
the " cloven flat disc " hypothesis, or that the structure 
of the visible universe is that of a flat disc, of irregular 
shape, with one of its halves cleft from the rim to the 
centre, and its breadth very much greater than its 
thickness. He placed our sun at almost the centre of 
the disc. 

He estimated the thickness of this flat disc to be 
eighty times greater than the distance of the Stars 
of the first magnitude, and its breadth twenty-three 
hundred times greater. 

As the foundation of his theory, Hersehel assumed 
the Stars to be uniformly distributed in space. 



The Starry Heavens. 201 

He afterwards gave up this assumption, and so aban- 
doned his hypothesis. 

He later invented another method of determining the 
arrangement of the visible Star system, based, however, 
on the assumption that the brightness of a Star affords 
an approximate measure of its distance. This assump- 
tion must be abandoned, although Herschel maintained 
it to the end. 

The most powerful telescopes have never been able 
to discern the smallest disc in a fixed Star. We are, 
therefore, totally ignorant of their real size. We can- 
not then say whether the greater lustre of a Star arises 
from its greater nearness, greater size, or the greater 
intensity of its light. 

The theories on the construction of the sidereal uni- 
verse ventured up to the present are of little moment, 
so much has to be assumed. 

The Magellanic Clouds. — The Magellanic Clouds are 
two large nebulous spots near the Southern Pole, and 
distinctly visible to the unassisted eye. 

They are situated in a region of the sky remarkably 
destitute of Stars, and are consequently very pictur- 
esque objects, being considered one of the wonders of 
the heavens. 

The larger and brighter cloud lies between the Pole 
and Canopus, the smaller and fainter one between the 
Pole and Achernar. 

The rays of the full moon render the lesser cloud 
invisible. 

The larger of the clouds is also called Nubecula 
Major, and the smaller Nubecula Minor. 

The great cloud covers a space of the sky of forty- 
two, and the lesser of ten, square degrees. 

These clouds are not a part of the Milky Way, with 



202 4 1 s tr oxo Mr : Xe w a xd Old. 

which they are not connected, nor indeed with one 
another. 

Sir John Herschel resolved the clonds with his twen- 
ty-foot reflector as follows : The Nubecula Major into 
582 Stars, 291 nebulae, and 46 clusters ; and the Nube- 
cula Minor into 200 stars, 37 nebulae, and 7 clusters. 

In no other part of the sky are so many stellar and 
nebulous masses thronged together in an equally small 
area as in the Nubecula Major. 

The first notice of these clouds is found in the writ- 
ings of a Persian astronomer of the tenth century, 
Abdurrahman Sufi, of Irak, who refers to the larger 
cloud as the White Ox. 

They received the name of Cape Clouds from the 
Portuguese, and the more familiar one, by which they 
are ordinarily known, from the first circumnavigator 
of the globe, the famous Magellan. 



CHAPTEE XIII. 
CELESTIAL PHOTOGBAPHY. 

The Principle of Photography — In the sunbeams 
there are heating, luminous, and chemically-active 
rays. These chemical or actinic rays possess the 
property of blackening the salts of silver, and parti- 
cularly the iodides and bromides. 

This property of light, which is the life of the photo- 
grapher's art, was known to the alchemists of the 
twelfth century. They had a preparation known as 
Luna Cornea, or Horn Silver (chloride of silver), of 
snowy whiteness, which was observed to - blacken by 
exposure to sunlight. 

Priestley and Scheele guided the infant steps, and 
Wedgwood, Davy, Mepce, Daguerre, and Talbot aided 
the growth of actinism. 

But the greatest name in photography is probably 
that of F. Scott Archer, who, in 1851, introduced the 
collodion process into the art. 

The Collodion Process. — A peculiarity of both ether 
and alcohol is that they evaporate rapidly when 
exposed to the air. 

Collodion is dissolved in a mixture of ether, alcohol, 
and cadmium iodide, or other soluble iodide. A glass 
plate is coated with this solution and exposed to the 
air. The ether and alcohol quickly evaporate, and a 
slender film of the collodion and iodide adheres to the 



The glass is then immersed in a bath of nitrate 
of silver, saturated with silver iodide; and the iodide 
of the film becomes iodide of silver, the cadmium being 
replaced by the silver. 

203 



204 Astronomy: New axd Old. 

By the exposure of the plate thus prepared to the 
sunbeams passing through the lens of a camera obscura, 
an invisible image of the surface reflecting the light 
rays is formed. 

The plate is then treated with a solution of pyrogallic 
acid, or of proto-sulphate of iron, mixed with a small 
quantity of acetic acid, and the image appears as a neg- 
ative, in which the lights and shadows answer respec- 
tive]) to the shadows and lights of the original. 

The image is fixed by pouring upon it a solution 
of cyanide of potassium, or steeping the plate in hypo- 
sulphite of soda. 

When the collodion film is exposed in the camera 
while still moist with the nitrate of silver solution 
it is called the wet collodion process, and is very sensi- 
tive to light rays. 

When the free nitrate of silver is washed off from 
the plate, and the film allowed to dry in the absence 
of light, it is called the dry collodion process, and 
is much less sensitive to the tithonic rays than the wet 
plate. 

First Steps.— Dr. Henry Draper, of New York, and Pro- 
fessor Bond, of Cambridge, made the first experiments 
in Celestial Photography. Warren De la Eue, how- 
ever, reached the first valuable results. 

In 1853 he took some good pictures of the moon 
by the collodion process, using a reflector of thirteen 
inches. 

In 1857 he obtained, by an almost instantaneous 
exposure, the first Solar Photograph of real value ever 
taken. 

The first really successful photograph of a solar 
eclipse was taken by him on July 18, 1860. 

Tebbutt's comet (comet 1881, III.) was the first one 



Celestial Photography. 205 

satisfactorily photographed. The actinic strength of 
cometary rays is exceedingly small. An exposure of 
about three days would be required to get a good 
impression of a comet by the wet collodion process. 

A new process had, however, been recently invented, 
by which the wet collodion plates were replaced by dry 
u gelatino-bromide " ones of extraordinary sensitive- 
ness. 

In these plates silver bromide is exclusively used 
as the sensitive substance, and has a reducing power 
of its own. 

Janssen, by this new process and the aid of a reflec- 
tor of great light-gathering power, obtained a photo- 
graph of this comet with half an hour's exposure. 

Dr. Draper also obtained a good photograph of this 
comet. 

Bond and Eutherfurd were the pioneers of Stellar 
Photography. 

Advances. — A fair photograph of the great nebula 
in Orion was taken by Draper in 1880, and Ainslie 
Common obtained a magnificent picture of this wonder- 
ful object, in 1883, from an exposure of thirty-seven 
minutes, with the assistance of a silvered-glass reflector 
of thirty-six inches. 

This is a miracle of the photographer's art. The 
most skilful artist could not by years of industry take 
as true a hand sketch of this nebula. 

Dr. Huggins first applied this dry-plate process 
of wonderful sensitiveness to celestial objects in 1876. 
A perfect picture can be obtained by this method from 
an exposure of the one-hundredth part of a second. 

Indeed, there is no limit to the briefness of time 
in which a good picture can be taken by this almost 
infinitely sensitive plate, if the light be sufficiently 



206 Astronomy: New axd Old. 

strong. And objects whose light-beams are of almost 
infinite feebleness can be photographed by continued 
exposure. 

A very faint object appears no brighter to the eye 
at the end of an hour than at the end of the first 
second. But the contrary is true of a photographer's 
plate. The effect accumulates almost indefinitely. 

This property of the plate is of priceless value to the 
astronomer. The plate that can be made sensitive 
enough to take a perfect impression of a bright object 
in the hundredth part of a second will take equally 
as good an impression of an object one hundred times 
as faint from an exposure of one second, and of 
another one thousand times as faint in ten seconds. 

By means of long exposures of sensitive plates 
photographs have been obtained of stars and nebulae 
absolutely invisible in the most powerful telescopes, and 
an hour's exposure of one of these plates will give a 
fuller and a more accurate star-chart than an observer's 
work of many years by the old methods. Celestial 
Photography is indeed advancing with rapid strides. 



CHAPTER XIV. 
CELESTIAL LAWS. 

Laws of Motion. — The three laws governing Motion 
were, in the main, discovered by Galileo and Huyghens, 
although Newton first presented them in a systematized 
way in his Principia. 

That a body will continue in the state in which it is, 
either of rest or of uniform rectilinear motion, unless 
acted on by some force to change its condition, is the 
first law of Motion or the principle of inertia. 

The second law of Motion is, that change in the direc- 
tion of motion is always proportioned to the force 
applied, and will take place in a right line with the 
impressed force. 

The third law of Motion is, that action and reaction 
are equal, and in opposite directions; or, the mutual 
actions of any two bodies are always equal, and oppo- 
sitely directed in the same straight line. 

These three great laws prevail throughout the uni- 
verse of matter, and have been established by experi- 
ment, aided by calculation. 

Laws of Gravity. — The intensity of the attraction of 
gravitation is in no way controlled by the nature of the 
substances drawing one another, but depends altogether 
on the magnitude of their masses. 

This force differs entirely from magnetic attraction, 
and is incomparably less intense, being only infinitesi- 
mal where small masses are concerned. 

It is computed £hat two solid cast-iron balls, about 
fifty -three yards in diameter, and one mile apart, will 



208 Astroxomy: New axd Old. 

attract one another, owing to gravitation, by a force 
equal to about one pound pressure. 

No screen nor intervening object intercepts the 
attraction of gravitation. 

Galileo discovered, by experiment and observation, 
the laws of terrestrial gravitation, and since his time it 
has been known that this force is inherent in the 
earth. 

The laws of terrestrial gravitation, as illustrated in 
falling bodies, may be concisely stated as follows: 
First, all bodies near the earth's surface fall in straight 
lines towards the earth's centre, or in lines perpendicular 
to the surface of still water. 

Secondly, all bodies, whatever their quantity of 
matter, must fall to the earth from the same height 
with equal velocities. 

Thirdly, bodies falling towards the earth under the 
influence of gravity have their motion constantly 
accelerated. 

Fourthly, the force of gravity is proportioned to the 
quantity of matter attracting. 

Fifthly, the force of gravity varies inversely as the 
square of the distance from the centre of the earth ; or, 
the attraction of gravitation is inversely as the square 
of the distance. 

Kepler's Laws. — John Kepler, of Wiirtemberg, after 
many years of assiduous observation, discovered the 
three great la \vs governing the motion of the planetary 
bodies around the sun. 

Kepler was a man of extraordinary perseverance, 
and although he repeatedly failed to make his theories 
agree with observation, still his profound conviction 
that God had established fixed laws to govern His 
world animated him to constantly fresh endeavors to 



Celestial Laws. 



209 



discover these laws, until he achieved one of the most 
glorious triumphs in the history of Astronomy. 

Kepler first chose a circle for the hypothetical path 
of the planets, but soon discovered that he could not 
reconcile observation with a circular path, and so aban- 
doned that form of curve. 

He next tried the ellipse, and found, on this hypoth- 
esis, that the theoretic and observed places of the 
planets closely agreed. 




Fig. 12.— Describing an Ellipse. 

The Ellipse. — An Ellipse is a conic section formed by 
cutting a right cone with a plane oblique to its sides. 
The characteristic feature of the curve is, that the sum 
of the distances of any point in it from two fixed points 
within is constant. The two fixed points are called the 
foci. 

A simple way of describing an ellipse is to fasten on 
a plane surface the two ends of a thread with pins, and 



210 



Astronomy: New axd Old. 



make a pencil move on the plane, keeping- the thread 
constantly stretched. 

The thread must be longer than the distance between 
the pins. 

The pins will be found at the foci. The axis of the 
curve running through the foci is called the Major 
Axis, and is equal to the length of the thread. 

The Eccentricity. — The eccentricity of an ellipse is 
the distance of its centre from either focus, and the less 
it is the closer docs the ellipse approach the circular 
form. 

The Radii Vectores. — The lines joining the foci with 

any point in the curve are called the Radii Vectores. 

The first law of the planetary motions is that the 

orbits of the planets are ellipses, having the sun at one 

of their foci. 

That a line joining the 
centres of the sun and a 
planet sweeps over equal 
areas in equal times, is 
Kepler's second law. 

Although the planet 
when in the part of its 
orbit nearest the sun 
moves more rapidly and 
passes over a greater 
arc in the same inter- 

Fig. 13.-Radii Vectores. yal of time tnan when 

it is farthest away, still the triangular space travelled 
over by the radius vector is always the same for equal 
times. 

Third Law. — The squares of the periodic times of the 
revolutions of the planets are proportioned to the cubes 
of their mean distances from the sun. 




Celestial Laws. 211 



Universal Gravitation — Newton generalized the laws 
of Gravitation by showing that they extended to the 
moon, the planets, and the sun. 

Newton knew that if the force of gravity reached 
from the earth to distant bodies, it should decrease 
as the square of the distance from the earth's centre 
increased. 

The moon is about sixty times as far as the earth's 
surface from the earth's centre, and the force of attrac- 
tion at the moon ought to be sixty squared times weaker 
than it is at the earth's surface. 

Newton made the computation, and found that the 
moon falls the one-twentieth of an inch from a tangent 
every second toward the earth, and thus demonstrated 
that our satellite obeys the law of gravitation. 

Newton's law is that every body of matter in the uni- 
verse attracts every other in direct proportion to its 
mass, and in the inverse ratio to the square of the dis- 
tance. 

The observations of the conduct of double stars show 
that this great law is also paramount in the stellar 
systems. 

Planetary Perturbations. — The first law of Kepler con- 
cerning the ellipticity of the paths of the planets around 
the sun is only rigorously true when the planets are 
regarded as immaterial points, and the sun truly spheri- 
cal or homogeneous. 

But the planets being really great massive bodies 
acting and reacting upon one another and upon the sun, 
the shapes and positions of their orbits undergo con- 
tinual though small changes, called Perturbations. 

These perturbations are of two kinds, periodic and 
secular. 

Periodic Perturbations. — The perturbations or varia- 



212 Astroxomy: New axd Old. 

tions produced in a planet's orbit by the disturbance of 
the other planetary bodies during its periodic time, or 
in the course of a single revolution, are known as peri- 
odic perturbations. 

Secular Perturbations. — The periodic perturbations con- 
tinually occurring form a series or cycle of changes, 
requiring great periods of time for their completion, and 
so are called secular perturbations or variations. 

Laplace and Lagrange have demonstrated that these 
perturbations do not endanger the stability of the plan- 
etary system, but are compensated by the return of the 
planetary orbits, after great lapses of time, to their 
former shape and position. 

Precession of the Equinoxes. — The Ecliptic, or sun's 
apparent path, crosses the earth's equator, to which 
it is oblique, in two points, called the Equinoctial Points 
from the fact that the nights and days are equal all the 
world over when the sun is in these parts of his 
path. 

There is a great bulge, or protuberance, around the 
equator which the sun constantly attracts towards it- 
self, or towards its own apparent path rather, the 
Ecliptic. 

The consequence of this attraction is a slight motion 
of the earth's equator toward the Ecliptic, or a forward 
movement of the equinoctial points along the Ecliptic. 
This is really a precession, or a moving forward, of the 
equinoxes. 

This motion of the earth's equator imparts a motion 
to the earth's axis, or an apparent motion to the celes- 
tial pole. Owing to precession, the celestial pole will 
make a complete revolution in 25,868 years. 

Nutation. — The moon also attracts this same equator- 
ial bulge, and gives it another and more rapid motion, 



Celestial Laws. 213 

by which the celestial pole makes an apparent revolu- 
tion in 18.6 years. 

This motion or nodding of the earth's axis is called 
Nutation. 

The moon's path crosses the earth's at two points, 
called the moon's nodes, and Nutation is equivalent 
to a precession of the moon's nodal points along the 
Ecliptic. 

Lunar Perturbations. — The sun's attraction on the 
moon causes variations in the lunar orbit. 

At new moon the moon is nearer to the sun than the 
earth is, and the moon is more attracted toward the 
sun than is the earth. At full moon the earth is nearer 
to the sun than the moon is, and the earth is more 
attracted than the moon. In both these cases the ten- 
dency of solar attraction is to separate the earth and 
moon. 

At the quarters the earth and moon are about equally 
attracted, but the direction of the attraction is not the 
same for both, and the effect of solar gravitation is to 
draw the earth and moon nearer to one another. 

Evection. — The earth, the moon, and the sun are in 
a line at full and new moon. This line is called the line 
of the Syzygies. 

When the moon is in the first and third quarters 
it is said to be at the quadratures. 

Consequently, the solar influence tends to increase 
the gravity of the moon to the earth at the quadratures, 
and to diminish it at the syzygies. 

When the moon is in that part of its orbit nearest 
to the earth, it is said to be in Perigee, and farthest 
from the earth, in Aj>ogee. 

The line joining the perigee and apogee is called the 
line of the Apsides. 



214 Astronomy: New and Old. 

The eccentricity of an elliptical orbit is the distance 
from the centre to either focus. 

The sun's attraction causes the moon's orbit to be 
most eccentric when the line of the apsides is in the 
syzygies, and least when the line of the apsides is in the 
quadratures. 

The perturbation of the moon's orbit, owing to this 
change of its eccentricity caused by solar attraction, 
is called the moon's Evection. 

Motion of the Apsides. — Another of the lunar perturba- 
tions is, that the line of the apsides, owing to planetary 
disturbance, makes a complete revolution in 8.8 years. 

This motion of the line of the apsides (the line joining 
the perihelion and aphelion points in planetary orbits) 
is common to all the planetary bodies, but is much less 
remarkable in their case than in that of the moon. 

The Tides. — Twice a day, at intervals of about twelve 
hours and twenty-five minutes, the sea-coasts present 
the well-known spectacle of the flow of the tide. 

The tide by degrees rises, gaining on the beach, 
which it covers to a greater and greater height ) and 
after six hours' swelling attains its maximum. 

Scarcely is the moment of high-water or flood-tide 
attained than the flow of the water ceases, the descent 
begins, and the ebb succeeds to the flow. 

Between two consecutive flood-tides there is an inter- 
val of about twelve hours and twenty-five minutes. 

Thus, from one day to another high water is about 
fifty minutes behind. 

The transit of the moon over the meridian is also 
about fifty minutes later each successive day. 

High water reaches the meridian fifty minutes later 
each successive day, and each successive day the moon 
crosses the meridian fifty minutes later. 

Nothing seems clearer than that high water and the 



Celestial Laws. 215 

moon travel along together. In twenty-nine and a half 
days, or the periodic time of a full revolution of its orbit 
by the moon, the tide is twenty-four hours late, and our 
satellite is also twenty-four hours behind, having in that 
interval made a complete circuit of the heavens. 

The moon, by the force of her gravity, draws the 
waters of the earth into an oval form towards her- 
self. 

The earth daily rotating on its axis brings some por- 
tions of its liquid mass nearer to the moon than other 
portions. The bodies of water nearer to the moon will 
be more strongly attracted by her than those more 
remote. 

The sun attracts the terrestrial waters similarly as 
the moon. 

The tides are caused by the unequal attractions of 
the sun and moon upon the waters of different parts 
of the earth. 

The moon, owing to her greater proximity, produces 
a greater tide than the sun, not, however, because her 
actual amount of attraction is thereby rendered greater 
than the sun's, but because her attraction for the differ- 
ent parts of the earth is very unequal, while that of the 
sun is nearly uniform. 

It is the inequality of gravitation, and not its real 
amount, that produces the tides. 

The sun is distant from the earth about 92,500,000 
miles, and the earth's diameter is only about the one 
twelve-thousandth part of this distance, so that the 
solar attraction will be nearly the same on all parts 
of the earth. 

But the earth's diameter is one-thirtieth of the 
moon's distance 5 and the moon acts, consequently, 
with considerably greater power on one part of the 
earth than on another. 



216 Astroxomy: New axd Old, 



It is computed that the tide-producing force of the 
sun and moon is equal to their mass divided by the 
cube of their distances. 

The sun is four hundred times more distant than the 
moon, and by an easy calculation it is found that the 
tide-producing force of the moon to that of the sun 
is as 5 to 2. 

At new and full moon the sun and moon act together 
on the waters, and produce the spring-tides, or "the 
tides of the syzygies." 

At the first and last quarter of the moon the sun and 
moon act in opposition on the waters and produce the 
neap-tides, or "the tides of the quadratures." 

The variations of the moon's distance from the earth 
influence the tides considerably. The tides are greater 
when the moon is in perigee than in apogee, and should 
the moon be in perigee during the syzygies the tides 
would be unusually high. 

The variation in the sun's distance does not materi- 
ally affect the tides. 

Another cause of change in the magnitude of the 
tides is the amount of the declinations (distance from 
celestial equator) of the sun and moon. 

The nearer these bodies are to the equator the higher 
proportionately will be the tides at the equator. 

The equinoctial spring-tides occur when the moon 
is near the equator, on March 21 and September 22. 

When the earth is attracted by the sun or moon, it is 
attracted altogether, and the force acts on its centre. 

The particles of water just under the moon are nearer 
to the moon than is the earth's centre, and these 
particles are more strongly drawn toward the moon 
than is the earth's centre, and consequently rise up 
producing a tide. 

The particles of water on the side of the earth away 



Celestial Laws. 217 

from the moon are less attracted than the earth's centre, 
so that the earth is drawn away from the water, caus- 
ing the water to rise on the land, producing a tide 
on the side of the earth opposite to the moon. 

Thus there are two tides at the same time, one on the 
side of the earth nearest the moon, and one on the 
side farthest away. On one side the water is pulled 
away from the earth, and on the other the earth is 
pulled away from the water. 

The tidal wave is an undulation, and not a progres- 
sive motion, unless where the water is very shallow. 

The tidal wave does not reach the meridian simulta- 
neously with the crossing of the moon, because the 
water is retarded by its inertia, and the friction of the 
bottom and the banks of the seas. 

The wind, the depth and extent of seas, the direction 
and configuration of coasts, influence the tides. 

Inland seas and great lakes have no perceptible tides, 
because their extent, though considerable in itself, 
is exceedingly small compared with the whole earth. 

New York has a tide of about five feet ; Boston, 
eleven ; Apple Eiver, fifty, and Fundy Bay, in New Cale- 
donia, nearly one hundred feet. 

It is thought that the tides act as a friction-brake 
upon the earth's rotatory speed, lengthening the day. 

On this hypothesis it is computed that, after the lapse 
of sufficient time, the day will grow to be as long as the 
month, and after a still much greater period will equal 
the year, our planet ultimately turning the same face 
always toward the sun. 

It is said that the lunar tides have acted similarly, 
but much more strongly, and so more rapidly, on the 
moon's rotatory velocity, until the lunar day now equals 
the month, and our satellite keeps the same face con- 
stantly toward its primary. 



CHAPTER XV. 

CELESTIAL MEASUREMENTS. 

Elements. — The precise position of a place upon the 
earth's surface is known by its Latitude and Longitude. 

The Latitude is the distance north or south of the 
equator, measured on the meridian of the place. 

Longitude is measured along the equator, and is the 
portion of that great circle intercepted between the 
meridian of a place and some assumed meridian. 

The elements determining the place of an object in 
the heavens are Right Ascension and Declination. 

Declination — Declination is the celestial element 
corresponding to terrestrial latitude, and is the distance 
of a heavenly body north or south of the Equinoc- 
tial. 

The Equinoctial is a great circle of the heavens, 
being the projection of the earth's equator upon the 
celestial sphere. 

Right Ascension — Right Ascension corresponds to 
our longitude, and is measured along the Equinoctial 
eastwardly from the point of its March intersection with 
the Ecliptic, or sun's apparent path. 

The Ecliptic is inclined to the Equinoctial at an 
angle of 23° 28', and intersects the latter in two points, 
called the Equinoxes. 

Right Ascension is reckoned from the Vernal or 
March Equinox, or first point of Aries, and is measured 
eastwardly through 360°. 

Right Ascension is never reckoned westwardly. 

Frequently the element employed instead of Decli- 
nation is the Polar Distance. 

218 



Cel es tta l Me a s urbmbnts. 219 

Polar Distance — The Polar Distance is measured 
from the Celestial Pole along a great circle vertical 
to the Equinoctial. 

The Polar Distance is the complement of the Decli- 
nation (90° — Declination), and is indeed the same 
element. 

Distance of an Inaccessible Object.— In computing the 
distances of the heavenly bodies it is necessary to 
invoke the aid of trigonometry to some slight extent. 

One of the very first steps in trigonometry is to 
determine the distance of an inaccessible object, having 
the length of a base line, and the angles at the ex- 
tremities of this base line subtended by the object 
and the line. 

Three parts of a triangle being given, the triangle 
can be easily solved, and the other elements found. 

The solution is still simpler if the triangle be a right- 
angled one. 

In finding the distances of celestial bodies the tri- 
angle usually employed is a right-angled one, as the 
horizontal i)arallax is mostly used. 

In solving a right-angled triangle it is sufficient 
to know the base and the angle it subtends. 

To find the distance of a heavenly body it is enough, 
then, to have a measured base line, and the angle sub- 
tended at -the body by this base line ; or, in reality, 
the magnitude of the base line as seen from the 
distant body. 

Parallax — This angle, or magnitude, is called the 
parallax of the body, and is really its apparent dis- 
placement when viewed from both ends of the base line. 

For instance, if the moon, when near to a fixed star, 
be viewed at the same instant by two observers four 
thousand miles apart, one of them will see it nearer 



220 



Astronomy: New axd Old. 



to the star than the other. The reason of this is, the 
moon, being comparatively close to the observers, will 
suffer an apparent displacement by being viewed from 
two positions so widely separated; while the fixed 
star will suffer no apparent displacement, owing to its 
almost infinite distance. 

This apparent displacement of the moon is its 



Figure 14. 



parallax for a base line of four thousand miles, or 
is the magnitude of four thousand miles as seen 
from the moon, and is considerable. 

The fixed star suffers no displacement, because four 
thousand miles seen from a fixed star is the merest 
point and has no magnitude whatever, 



Celestial Measurements. 221 

The nearer a body is the greater is its parallax ; and 
the more distant, the less the parallax. 

When A and B (Fig. 14), a thousand miles apart, ob- 
serve at ths same moment a heavenly body, C, it will 
appear to A thrown back on the sky at F, and to B at 
K. The parallax, or apparent displacement of 0, viewed 
from A and B, is the arc, K F, or angle, A C B. 

When the object is more distant its apparent dis- 
placement is less, as represented by the angle, A D B, 
or arc, J G. 

When the heavenly body reaches E its parallax 
is still less, and is measured by A E B or I H. 

Parallax, in general, then, is the apparent dis- 
placement of an object as seen from different positions. 

Diurnal or Geocentric Parallax — The daily or geocen- 
tric parallax of a body is the apparent displacement 
it suffers when seen from the surface and from the 
centre of the earth. 

When the heavenly body, B (Fig. 15), is fa the zenith 
of the observer, A, it will have no daily or geocentric 
parallax, as it will appear at D both to A and C. 

When the observer, A, views the body, E, it will 
appear on the sky at F ; but when seen from the centre 
of the earth it will appear at G. The arc G F is the 
apparent displacement of the body, E, as seen from 
A and 0, or is its daily or diurnal parallax. 

Horizontal Parallax. — The horizontal parallax of a 
heavenly body is its apparent displacement caused by 
the whole radius of the earth, and is its greatest geo- 
centric parallax. 

Thus A, in the figure, will see the body, H, in his 
horizon at I; but from C it will appear at K, and its 
horizontal parallax is I K, or the angle C H A, being 
the magnitude of the earth's radius as seen from H. 



222 



Astronomy : New axd Old. 



Diurnal Method of Parallaxes. — Finding the parallax 
by means of the daily revolution of the earth is called 
the "Diurnal Method of Parallaxes/' and is the one now 
commonly employed. 

The moon's position relatively to a fixed star is 
noticed, and after the earth passes through a space of 
six hours, is again noticed. 




Figure 15. 



The difference of these distances from the fixed star, 
allowance being made for the moon's own motion and 
refraction, will give the moon's horizontal parallax. 

The moon's horizontal parallax being known, to find 
its distance is, as already mentioned, solely the solution 



Celestial Measurements. 223 

of a right-angled triangle, the base and opposite angle 
being given. 

This method of finding the distances of the heavenly 
bodies by the geocentric parallax is confined to the 
Moon, Mars, and the Minor Planets. 

For farther distances the earth's radius has no prac- 
tically appreciable parallax. 

Finding the Sun's Distance. — The horizontal parallax 
of the stm is so extremely small that it cannot be 
obtained directly with any accuracy, owing to the great 
refraction or bending of the light-rays near the hori- 
zon. 

Many indirect methods have been resorted to by 
observers, however, to determine it. 

The orbits of the planets are so connected by Kep- 
ler's laws that if the interval dividing any two of them 
can be accurately ascertained, the sun's distance can 
easily be deduced. 

One of the indirect methods of finding the sun's par- 
allax is by the observation of the transit of Venus across 
the sun's disc. 

When two observers on different parts of the earth 
view the transit of the planet, they will see Venus 
thrown against the face of the sun as a black spot, but 
not in the same place. 

The planet will not appear to enter on the sun's disc 
at the same absolute moment at the two stations, and 
therefore the paths, or chords, traversed will be dif- 
ferent. 

The chords will be of unequal lengths, so that the 
time of transit at one station will be different from the 
time of transit at the other. 

The difference will enable the observers, when they 
come together and compare notes, to determine the dit- 



224 Astroxomv: New and Old. 

ference in the lengths of the chords described by the 
planet, and consequently their respective positions on 
the solar disc, and so the amount of their separa- 
tion. 

The measure of this separation, or the apparent 
displacement of Venus on the sun's disc, is what is 
required. 

It is almost utterly impossible to obtain the measure 
of this displacement with sufficient accuracy, owing to 
the errors of refraction caused by the atmospheres of 
Venus, the Earth, and the Sun. 

A great drawback with this method, too, is the 
extreme rarity of the transits. 

The best and most available way, however, of finding 
the sun's distance is the diurnal method of taking suc- 
cessive morning and evening observations from the 
same spot, and by the same person, upon the Minor 
Planets, the rotation of the earth supplying the neces- 
sary difference in the points of view. 

This method is the one now most used, and has 
a number of advantages. About a dozen of these small 
bodies are sufficiently bright and approach near enough 
to the earth when on the same side of the sun with it 
(opposition) for this purpose. 

One very great advantage here enjoyed is that 
the planetoids have no sensible discs, appearing sim 
ply as points of light, and thus favor exact measure- 
ment. 

Another advantage is the unity of performance. 
Many small errors are avoided in the work being done 
by a single pair of eyes, and with the same optical 
instruments. 

These oppositions occur, too, frequently, and no elab- 
orate preparations are necessary for the work. 



Celestial Measurements. 225 

Heliocentric or Annual Parallax. — The Heliocentric or 
Annual Parallax of a heavenly body is its apparent dis- 
placement as seen from the earth and the sun, or is the 
magnitude of the radius of the earth's orbit as seen 
from the distant body. 

This base line is 92,500,000 miles, and viewed from 
the distance of most of the fixed stars, has no magnitude 
whatever. 

Stellar Parallax. — To find stellar distances this helio- 
centric or annual parallax is alone available. 

There are principally two methods employed in deter- 
mining the annual parallax of stars, the absolute and 
the relative or " differential." 

In order to obtain the absolute parallax of a star, its 
polar distances are regularly measured for a long period, 
and from the mean of all the observations the disj)lace- 
ment due to parallax is computed. 

The errors in this method arising from the variation 
in the refracting powers of the atmosphere, and from 
the expansion and contraction of the meridian circle or 
transit instrument by changes of temperature, would 
almost entirely vitiate so small a quantity as the paral- 
lax of a star. 

In the differential method, by which most of the 
stellar distances now known have been computed, the 
observer chooses two stars so close together as to be in 
the same field of view of his telescope. 

One of these stars is known from its proper motion to 
be much nearer than the other. The separation of the 
stars caused by the earth travelling around its orbit is 
measured as accurately as possible with the micrometer, 
and half the sum is the relative or differential parallax. 

In this case the parallax of the more distant star is 
assumed to be zero. 



226 Astronomy: New and Old. 

In the differential method the effects of retract inn 
may be said to be eliminated, as both the stars arc 
displaced through retraction l>y almost the same 
amount. 

Magnitude of Celestial Bodies. — The magnitude of any 
heavenly body having a sensible disc can be easily 
determined when its distance is known. 

The apparent diameter of the distant body is meas- 
ured with as much precision as possible, and divided by 
two to obtaiu the apparent radius. 

We thus have a right-angled triangle, with the hypo- 
thenuse, which is the body's distance, and the angle 
opposite the perpendicular, which is the apparent radius. 
to find the perpendicular. 

When the radius of a spherical body is known, its 
other dimensions can be ascertained. 

Refraction — It is a well-known principle in optics 
that when a ray of light passes from a rare to a more 
dense medium it is bent towards the perpendicular or 
vertical. 

The matter composing our atmosphere grows more 
and more dense as the earth's surface is approached. 

The light-rays coming in from space in an oblique 
direction are therefore constantly bent more and more, 
until they take the form of a curve, and we see the 
object in the direction of a tangent to this curve. 

This atmospheric refraction affects alike the light- 
rays from tlie most distant as well as nearer bodies. 

The waves of light from bodies in the Zenith, or 
directly overhead, are not in the least influenced by 
refraction, as they are already in the line of the ver- 
tical. 

The nearer bodies are to the horizon the more 
strongly is their light refracted, because its beams are 



Celestial Measurements. 227 

most oblique to the vertical, and must journey through 
the greatest stretch of atmosphere. 

The amount of refraction at the horizon is thirty-four 
minutes ; at a height of one degree it is but twenty-four, 
and at an elevation of forty-five degrees it is only about 
one minute. 

Thus, in observing the position of the heavenly 
bodies correction must be made for refraction, and it is 
of the utmost importance that the body be observed 
when as high as possible above the horizon. 

As the mean apparent diameters of the sun and moon 
are respectively thirty -two and thirty-one minutes, and 
the refraction at the horizon thirty-four, it follows that 
these bodies are entirely visible both after actually 
setting and before rising. 

It is refraction that gives to the sun and moon their 
oval shapes when close to the horizon. 

Refraction increases so rapidly towards the horizon 
that the lower limbs of these luminaries are raised by 
its action much more than the upper ones, and their 
discs consequently assume an oval figure. 

In most cases the contraction of the vertical diameter 
of the sun amounts to six minutes of arc. 

Twilight. — Twilight has an atmospheric origin, and 
is occasioned by the refraction and reflection of sunlight 
in its passage through air. 

The phenomenon, however, is principally due to re- 
flection. The air reflects to as some portion of light 
when the sun is as far below the horizon as eighteen 

degrees. 

At the equator the sun ascends or descends through 
eighteen degrees in one hour and twelve minutes. 

At the equator the path of the sun is Dearly perpen- 
dicular to the horizon, and it travels over this s\*avc 



228 Astronomy: New axd Old. 

rapidly ; but when the sun's path is more oblique it 
takes the luminary a longer time to pass over the space 
that carries it eighteen degrees below the horizon, and 
the twilight is lengthened. In one case the sun goes 
vertically down, and in the other it moves along a 
slant. 

Near the poles of the earth the sun is within eighteen 
degrees of the horizon during two-thirds of the year, 
and so these polar regions enjoy an almost perpetual 
twilight. 

The peculiar property the air has of dispersing light 
in all directions is what renders objects on the earth out 
of direct sunshine visible to us. 

Were it not for this property of the atmosphere, 
everywhere beyond the reach of the direct rays of the 
sun would be as dark as night. 

A fair degree of humidity increases the transparency 
of the atmosphere. 

When, particularly in hot weather, the stars look 
unusually bright, remote objects appear uncommonly 
distinct, and the sky's azure is unwontedly deep, it is 
the herald of approaching rain. 

Aberration. — The aberration of light is the apparent 
displacement of a heavenly body caused by the com- 
bined effects of the earth's motion and the velocity of 
light. 

Light travels very rapidly, but still requires time for 
its passage. During the time occupied by the light-ray 
in reaching us from a heavenly body the earth travels 
some distance in its path. 

The true place of a star is not where we observe it, 
but at a slight angle in the direction of the earth's 
motion. 

The amount of aberration is the proportion between 



Celestial Measurements. 229 

the respective velocities of the earth and light, and is 
very small indeed. Light is nearly ten thousand times 
more rapid than the earth in its flight. 

This apparent alteration of a body's place due to 
aberration is then extremely small ; still, in delicate 
measurements it must be allowed. 

The heavenly body returns to its original position 
at the end of a year, or revolution of the earth in its 
orbit. 

The phenomenon was discovered by Bradley in 1729 
while endeavoring to find stellar parallaxes. 

Latitude. — The Latitude of a place is its distance 
north or south of the Equator, measured on the arc of 
a great circle passing through the yjlace and the Pole, 
and perpendicular to the Equator. 

Latitude is reckoned from 0° to 90° ; the latitude 
of the Equator being 0°, and of the Pole 90°. 

When an observer is on the Equator the Pole 
is in his horizon, and if he travels one degree north- 
ward from the Equator, the North Pole will rise one 
degree above the horizon. 

The latitude of an observer is then represented by 
the elevation of the Pole above the horizon. 

If the Polar Star were situated exactly at the celes- 
tial Pole, its altitude as seen from any place would 
be the latitude of the place, and to find the latitude 
it would only be necessary to determine the star's 
elevation. 

The Polar Star is not, however, precisely at the 
celestial Pole, but makes a small circle daily around it. 

By observing the greatest and least meridian altitudes 
of the Polar Star, or, for that matter, of any circumpolar 
star, and taking half the sum, we obtain the latitude 
of a place, correction being made for refraction. 



230 Astronomy: New and Old. 

Longitude — Longitude is reckoned along the Equator, 
being the portion of that great circle intercepted 
between the first assumed meridian and the meridian 
of the place. 

The Meridian of a place is a great circle of the heav- 
ens passing through both celestial poles, and. through 
the zenith of the place. 

With us the First Meridian is sometimes assumed 
to be Washington, and sometimes Greenwich. 

The Longitude is counted 180° east and. west of the 
first meridian. 

Longitude may be reckoned in time as well as in 
degrees. 

The earth rotates through 3G0° in twenty-four hours, 
and so 15° are equal to one hour, and one degree to four 
minutes. 

If we know the difference in time between the first 
meridian and our own, we can easily reckon our Longi- 
tude by adding one degree for every four minutes if we 
are west, or subtracting if we are east, of the assumed 
first meridian. 

The difference of time between the places can be 
determined by means of the electric telegraph. The 
observers having, by the aid of transit instruments, per- 
fectly corrected their clocks, can, by a telegraph signal, 
immediately perceive the difference in local time be- 
tween the places, and make the computation accord- 
ingly. 

There are a number of other ways of finding the 
Longitude of a place with the aid of the Kautical 
Almanac, such as by Solar and Lunar Eclipses, Lunar 
distances, Occultation of Stars, and Eclipses of the 
Satellites of Jupiter. 



CHAPTER XVI. 

MECHANISM OF THE WOBLD. 

" God of the rolling orbs above ! 

Thy naine is written clearly briglit 
In the warm day's uu varying blaze 

Or evening's golden shower of light j 
For every tire that fronts the sun, 

And every spark that walks alone 
Around the utmost verge of heaven, 
Was kindled at Thy burning throne." 

— Peabody. 

GrOD is, indeed, plainly visible in His creation. On 
all sides, everywhere, are the most palpable evidences 
of omnipotence and infinite wisdom. Truly "the 
heavens declare the glory of God, and the firmament 
showeth His handiwork." 

Who bnt onr Creator conld impose upon inert matter 
the beautiful and sovereign laws of gravitation and 
motion ? Who but He could endow light and heat and 
electricity with their wondrous properties ? 

In the mighty magnitudes of the heavenly bodies, 
as in their awful distances and velocities, what omnipo- 
tence ! What wisdom in the singularly striking adap- 
tation of means to their ends everywhere apparent in 
creation ! 

The distance of the earth from the sun is just the 
most fitting for the best growth of our animals and 
plants, confining the heat of the sun within narrow and 
suitable bounds. 

The change of seasons is just precisely sufficient 
to produce a proper variety of animal and vegetable 



232 A 8 TR KOM Y : Xe w a xd Old. 

life. What could be more happy than the division of 
day and night . ; 

The tides are just within the limits to preserve the 
waters from corruption and not to overflow and deluge 
the earth. 

Look at the multiple beneficent uses of the atmos- 
phere. It supports the life of animals and plants, the 
one by supplying oxygen, the other carbon. What 
would destroy the animal vivifies the plant. 

it supports combustion. It tempers the heat and the 
cold. It prevents, by its pressure, all fluids on the 
earth's surface from passing immediately away in vapor. 
It nourishes the earth with rain, diffuses the sunbeams, 
and conducts sound. 

What a nice adjustment of the solar system for the 
maintenance of its stability! If the orbits of the great 
planets had been more eccentric or more inclined, the 
system would fall to pieces. Did all not revolve in 
the same direction, it would be fatal to stability. 

The orbits of Mercury, Mars, and the planetoids are 
much more eccentric than those of the great planets. 
The eccentricity of the paths of Mercury, Mars, and 
the planetoids can work no mischief to the system owing 
to their small masses. 

Ead the paths of the great planets been equally 
eccentric, serious derangement would result; the eccen- 
tricity of the earth's orbit would be gradually increased 
and the nature of its year completely changed; the 
moon might be precipitated upon us, or the planets 
might approach very near and draw the earth away 
from the sun. "We might have years of unequal 
length, and seasons of capricious temperature,- planets 
and moons, of portentous size and aspect, glaring and 
disappearing at uncertain intervals ; tides, like deluges, 



Mechanism of the World. 233 

sweeping over whole continents^, and perhaps the col- 
lision of two of the planets and the consequent destruc- 
tion of all organization on both of them." 

Nebular Hypothesis. — Concerning the arrangement or 
Mechanism of our system of worlds two opinions are 
worthy of consideration. 

One is that the Solar System, and, indeed, the Cos- 
mos? were created in their present shape instantly, out 
of nothing. 

The second is that the present form is the result of 
an arrangement of materials which were before "with- 
out form and void." 

There are a number of hypotheses purporting to 
account for the present harmonious mechanism of the 
planetary scheme. 

Among them the most beautiful and famous is the 
Nebular Hypothesis of Laplace. The nebular hypothe- 
sis treats only of the transformations, and does not 
concern itself with the origin of matter. 

Laplace begins by supposing the sun to have a more 
or less dense nucleus, surrounded by a rare, elastic 
atmosphere of vast extent. 

He considers this nucleus as either solid or so dense, 
compared with the atmosphere, as to be relatively solid, 
and to contain by far the greatest amount of the body's 
mass. 

He assumes, for the sake of convenience, the form of 
this nucleus to be already reduced to that of a spheroid, 
differing but slightly from a sphere ; but the shape of 
the atmosphere's bounding surface he leaves to be 
determined solely by the resultant of the centrifugal 
and gravitating forces, springing from any given mass 
and velocity of rotation that the body can have. 

The nucleus and atmosphere are rotating on an 



234 Astronomy: New axd Old. 

axis. Laplace calls the distance of that portion of the 
atmosphere from the axis where the centriftigal force 
just balances gravity, the Centrifugal Limit. 

Laplace then demonstrates mathematically that at 
the centrifugal limit of the atmosphere of a rotating 
body, over the equator, the equatorial radius is to the 
polar precisely as '•> to 2. 

When, then, the axial motion of the sun became 
so great that centrifugal force caused its atmosphere's 
equatorial axis to l>e to its polar as '"> to 2, the outer 
portion if the atmosphere would leave the sun. 

Laplace supposed that owing to excessive heat the 
atmosphere of the bud extended beyond the orbits of all 
the planets, and that it has successively contracted 
up to its present Limits. 

He conjectures that the planets were formed at the 
successive centrifugal Limits of the solar atmosphere 
by the condensation of the zones of vapor which, in 
cooling, it had been obliged to abandon in the j)lane 
of its equator. 

"The atmosphere of the sun," he says, "could not 
extend outward indefinitely. Its limit is the point 
where the centrifugal force, due to its axial motion, 
balances gravity. 

u Now, iu proportion as its cooling causes the atmos- 
phere to contract and to be condensed towards the 
sun's surface, the motion of rotation must increase. 
For, by virtue of the principle of areas, the sum of the 
areas described by the radius-vector of each molecule 
of the sun and of its atmosphere, when projected on the 
plane of his equator, being always the same, the rota- 
tion ought to be more rapid when these molecules are 
brought nearer the sun's centre. The centrifugal force, 
due to this increased motion, thus becoming greater, the 



Mechanism of the World. 235 

point at which gravity is equal to it approaches nearer 
the sun's centre. 

" By supposing, therefore, what it is very natural 
to admit, that the sun's atmosphere at any epoch had 
extended up to this limit, it would be necessary, on 
further cooling, for the atmosphere to abandon the 
molecules situated at this limit and at the successive 
limits produced by the increase of the sun's rotation. 

"These molecules, thus abandoned, have continued 
to circulate around the sun in the same direction 
as before, since their centrifugal force was just balanced 
by their gravity towards the sun. 

" But this equality of centrifugal force and gravity 
not taking place with regard to the atmospheric mole- 
cules placed on the parallels to the solar equator, these 
latter molecules, by their gravity, will follow the atmos- 
phere in proportion as it is condensed, and will not 
cease to belong to it until by their motion they have 
reached the equator. 

" Let us consider now the zones of vapor successively 
abandoned. These zones ought, most probably, to form 
by their condensation and the mutual attraction of their 
molecules, various concentric rings of vapor revolving 
around the sun. The mutual friction of the molecules 
of each ring ought to accelerate those moving more 
slowly, and retard the swifter, until they should all have 
acquired the same angular motion about the sun. 

" Hence, the real velocity of the molecules farthest 
from the sun will be the greatest. 

" The following cause ought to contribute also to this 
difference of velocity. The molecules of the ring most 
distant from the sun, and which, by the effect of cooling 
and condensing, are brought nearer, so as to form the 
outer portion of the ring, have always described areas 



236 Astronomy: New and Old. 

proportional to the time; since the central force by 
which they are animated has been constantly directed 
towards the Bun's centre. 

"Now, this constancy of areas requires an increase 
of velocity in proportion as they approach the centre 
of motion. It is evident that the same cause ought to 
diminish the velocity of those molecules which, by the 
cooling and contracting process, are carried outwards to 
form the inner part of the ring. 

" If all the molecules of one of these vaporous rings 
had continued to condense without separating, they 
would have formed at last a liquid or a solid ring. 

k - But the regularity which such a formation requires 
in all parts of the ring, and in their rate of cooling, 
ought to render this phenomenon extremely rare. 

" Hence the Solar System offers but a single exam- 
ple of it; namely, that of the rings of Saturn. Almost 
always each vaporous ring ought to be broken into 
several masses, which, moving with nearly the same 
velocity, have continued to revolve around the sun at 
the same distance from him. 

u These masses ought each one to take on a 
spheroidal form, with a motion of rotation in the same 
direction as their motion of revolution around the sun; 
since their molecules nearest to him had less velocity 
than those farthest from him. 

tt They must, therefore, have formed so many planets 
in a vaporous condition. But if one of them had been 
large and powerful enough to successively reunite by its 
attraction all the others around its own centre, the vapor- 
ous ring will have been thus transformed into a single 
spheroidal vaporous mass revolving around the sun 
nearly in the plane of his equator, with a nearly circular 
orbit, and with its motion of rotation generally in the 



Mechanism of the World. 237 

same direction with that of its revolution around the 
sun. 

" This last case has been the most common ; but the 
solar system offers to us an example of the first case in 
the four small planets revolving between Mars and 
Jupiter, unless we suppose, with Olbers, that they 
formed at first a single planet which some strong explo- 
sion has divided into several parts, animated by different 
velocities. 

"If, now, we follow the changes which further cooling 
ought to produce in the planets consisting of vapor, the 
formation of which we have just considered, we shall see 
a nucleus begin at the centre of each of them, and see 
it grow continually by the condensation of the atmos- 
phere which surrounds it. 

" In this state the planet perfectly resembles the sun 
in the nebulous condition which we have been consider- 
ing. Its cooling ought, therefore, to produce, at the 
different centrifugal limits of its atmosphere, phenomena 
similar to those which we have described ; that is to say, 
rings and satellites revolving around its centre in the 
direction of its motion of rotation, and the satellites 
rotating also in the same direction on their axes. 

"The regular distribution of the mass of Saturn's 
rings around his centre, and in the plane of his equator, 
results naturally from this hypothesis, and without it 
becomes inexplicable. These rings appear to me to be 
the ever-existing proof of the former extension of 
Saturn's atmosphere, and of its successive contractions. 

" Thus, the singular phenomena of the small eccen- 
tricities of the orbits of the several planets, and those of 
their satellites, or their almost circular orbits, the small 
inclinations of these orbits to the sun's equator, and the 
identity of the motions of rotation and revolution of all 



238 , 1 s ri? o xom y : Ne w a xd Old. 

these bodies with that of the sun's rotation, flow from 
tlie hypothesis which we propose, and give to it a great 
probability, which may he still further increased by -the 
following considerations : 

" All the bodies which revolve around a planet, hav- 
ing been formed, according to this hypothesis, by the 
zones which its atmosphere has successively abandoned, 
and the planet's motion of rotation having become more 
and more rapid, the duration of this rotation ought to 
be less than those of the revolution of these different 
bodies. This must be true, likewise, for the sun in com- 
parison with the planets. All this is confirmed by 
observation. 

" The duration of revolution of Saturn's nearest ring 
is, according to Eerschel's observations, 0.438d., and 
that of Saturn's rotation is 0.427d. The difference, 
ii.oild.. is small, as it ought to be; because the part of 
Saturn's atmosphere which the loss of heat has con- 
densed upon the planet's surface since the formation 
of this ring being small, and coming from a small 
height, it ought to have produced but a small increase 
of the planet's rotation. 

" If the Solar System had been formed with perfect 
regularity, the orbits of the bodies which compose it 
would have been perfect circles, whose planes, as well 
as those of the different equators and rings, would have 
coincided exactly with the sun's equator. But we can 
conceive that the innumerable varieties which ought to 
have prevailed in the temperature and density of the 
several parts of these great masses have produced the 
eccentricities of their orbits and the deviations of their 
motions from the inane of the sun's equator. 

"In our hypothesis the comets are strangers to the 
planetary system. Considering them, as we have done, 



Mechanism of the World. 239 

as small nebulae wandering from one solar system to 
another, and formed by the condensation of nebulous 
matter so profusely scattered throughout the universe, 
it is evident that when they arrive at that part of space 
where the sun's attraction predominates, he compels 
them to describe elliptical or hyperbolic orbits. But 
their velocities being equally possible in all directions, 
they ought to move indifferently in all directions, and 
under all inclinations to the ecliptic, which is conform- 
able to observation. Thus the condensation of nebulous 
matter, by which we have explained the motions of rota- 
tion and revolution of the planets and satellites in the 
same direction and in planes of small inclination to each 
other, explains equally why the comets depart from this 
general law." 

Eeasoning backward from the point where he 
assumed, for convenience, the sun to be a dense nucleus 
with a hot extensive atmosphere, Laplace supposes the 
radiant orb, in a more primitive state, to resemble those 
nebulae shown by the telescope to be composed of a bril- 
liant nucleus surrounded by a nebulosity which, by con- 
densing towards the surface of the nucleus, transforms 
it into a star. 

Judging from analogy, he supposed the stars all 
formed in this way by condensation from nebulous 
matter. Each condition of nebulosity was preceded 
by other conditions, in which the nebulous substance 
was more diffused, and the nucleus less luminous and 
less condensed. In this way he reaches a condition 
of nebulosity barely existing. 

Because our planets and satellites are the offspring 
of the same atmosphere in whose primitive motion all 
partook, Laplace points out as proofs of the truth of his 
hypothesis; that the movements of the planets are 



240 Astronomy: New and Old. 

all in the same direction, and nearly in the same 
plane ; 

That the motions of the satellites are in the same 
direction as those of the planets ; 

That the rotations of these different bodies, and 
of the sun, are in the same directions as their orbital 
mot ions, and in planes that vary but little from each 
other ; 

That the paths of both planets and satellites are 
nearly circular, or of small eccentricity ; 

That, contrarily, the orbits of comets are of great 
eccentricity, and of every inclination to the ecliptic, and 
that their motions are in all directions. 

Laplace puts forth his hypothesis with much diffi- 
dence, as the merest speculation. 

Shortcomings of the Hypothesis. — Laplace assumes 
nearly everything. Be does not explain the origin 
of the primitive nebula, nor how it received its first 
motion. 

Be fails to show how the primitive solar atmosphere 
came by just such fitting properties of not being too 
viscous nor too fluid, but just of the proper consistency 
and elasticity to fly off under the influence of a certain 
amount of velocity. Nor does he give any reason why 
tliis atmosphere condensed into beautiful planets in- 
stead of meteoric stones, as would be the more likely ; 
nor how it came to be endowed with suitable degrees 
of cohesion and adhesion, and the properties requisite 
for contraction and condensation. 

Laplace likened the Solar System in its primitive 
state to the distant nebulae, which he looked upon 
as forming star systems, and external to, and quite 
distinct from, the sidereal universe. 

But these nebulas are not external galaxies, nor 



Mechanism of the World. 241 

distinct from the sidereal system, but are indeed part 
and parcel of it. 

From the examination of the great irregular nebula 
surrounding Eta Argus, the great Orion nebula, the 
nebulae of the Nubecula^ and similar nebulae, it cannot 
be doubted that a real and close association exists 
between the stars and nebulae, and that they really 
constitute but a single system. 

According to Laplace, the primary must rotate on 
its axis in less time than its satellite revolves about it. 

The inner satellite of Mars, on the contrary, revolves 
about him three times while he is rotating once. 
Here is an observed fact, opposing the hypothesis. 

The sun has by tidal action somewhat retarded the 
axial velocity of Mars, but certainly not to this extra- 
ordinary extent. 

It is admitted that the earth's axial motion has been 
but little affected by solar tidal action. Solar tides 
on Mars could not, then, have produced such wondrous 
effects. 

If the mass of Mars be less than the earth's, his 
diameter is also much less, and, other things being 
equal, tidal action is proportioned to the diameter 
of the body acted upon. Mars, too, is one and a half 
times more distant than the earth from the sun. 

One of the main pillars of Laplace's hypothesis 
is the uniformity of the motions, both axial and revo- 
lutionary, of the planets and satellites in the same 
direction from west to east. Here again is an observed 
fact against the hypothesis. The satellites of Uranus, 
and that of Neptune, are known to have a retrograde 
movement. 

There is a great dynamical principle known as the 
conservation of the "moment of momentum," This 



242 4 1 s TR onom ) ' : Ne w a xd Old. 

conservatioD of the moment of momentum differs en- 
tirely from what is known as the conservation of energy. 

The energy of the solar system can be transformed 
into heat, and a portion of it constantly dissipated and 
lost in space, but no action of the system itself can 
ever alienate a single iota of the moment of momentum. 

The relative distribution of the moment of momen- 
tum may be altered, but the total amount, barring 
external influence, can never be changed. 

If we multiply Jupiter's mass by his angular orbital 
motion in one second, and the product by the square 
of his distance from the sun, we obtain Jupiter's orbital 
moment of momentum. 

It' we multiply Jupiter's mass by his angular rota- 
tory motion in one second, and the product by the 
square of ;t line depending on his constitution, we have 
his rotational moment of momentum. 

Similarly the moments of momentum of the other 
planets are deduced. 

If we multiply the sun's mass by his angular rota- 
tory motion in one second, and the product by the 
square of a line depending on his constitution, we obtain 
his rotational moment of momentum. 

Professor Ball gives the following distribution of the 
moment of momentum in the Solar System, the total 
being taken as 100: 

Orbital moment of momentum of Jupiter 60 

Orbital moment of "momentum of Saturn 24 

Orbital moment of momentum of Uranus 6 

( hiatal moment of momentum of ^Neptune 8 

Rotational moment of momentum of the Sun. . 2 

Total 100 



Mechanism of the World. 243 

The otlier bodies are not considered, their moment 
of momentum being comparatively infinitesimal. 

Professor Ball says : "Jft might be hastily thought 
that, just as the moon was born of the earth, so the 
planets were born of the snn, and have gradually 
receded by tides into their present condition. We have 
the means of inquiry into this question by the figures 
just given, and we shall show that it seems utterly 
impossible that Jupiter, or any of the other planets, can 
ever have been very much closer to the sun than they 
are at present." 

Above all, it seems utterly impossible that Jupiter 
could have received his orbital moment of momentum 
from the sun. 

Laplace's hypothesis places the centrifugal limits 
of the abandoned portions of the revolving glowing 
atmosphere of the sun widely apart. After abandoning 
the first vaporous ring, the atmosphere contracts to 
nearly one-half its primitive bulk before throwing 
off another. The abandonment of each ring was fol- 
lowed by an immense atmospheric shrinkage. 

This would demand such great cohesion in a glowing 
mass of vapor as it is difficult to concede it possessed. 
It would seem to be more in accord with the character 
of a gaseous body that, when the centrifugal limit was 
reached the first time, the outer mass, under the influ- 
ence of centrifugal force, would partially separate from 
the portions next to it; then these would separate 
next, and so on. In this way, instead of a series of 
rings, there would be a constant dropping off of matter 
from the outer portions, producing an almost infinite 
number of concentric rings, all joined together. Thus, 
there would result a meteoric instead of a planetary 
system. This is the objection of Professor Kirk wood. 



244 Astroxomy: New axd Old. 

Professor Newcoinb considers that the rings were all 
thrown off together, and that the inner and .smaller 
bodies are, if anything, the older. 

Faye thinks that the outer planets were formed 
last. 

Thus it appears that Laplace's hypothesis is lar 
from being established, if indeed it has not altogether 
failed. 



INDEX. 



PAGE 

Abdurrahman 202 

Aberration 83, 228-229 

Aberration, Secondary 84 

Absorption, Theory of 45 

Acetic Acid 204 

Acetylene 134 

Achernar 179, 201 

Achromatism 83 

Actinic Strength, The 205 

Adams 70, 117, 143, 144 

Adams Prize, The 112 

^Egean, The 169 

Aerolites 146 

Aerosiderites 146 

Aerosiderolites 148 

Airy, Sir Geoige 63, 192 

Aladdin 1 123 

Albategnius 8, 16 

Alchemist j, The 203 

Alcohol 203 

Aldebaran 16, 160, 170, 171, 179 

Alexander the Great 12 

Alexandria 12, 15, 16, 32 

Alexandrian Astronomers 13 

Alexandrian School 12, 13 

Alfonso X 17 

Algol 161, 184, 185 

Almagest, The 15, 16 

Almanac, The 16 

Almanacs 27, 58 

Alpha Ceutauri... 176, 178,181, L82, 
L91, 198 

Alpha Cmcifi 179 

Alpha Cygni 168, 171, 172 

Alpheta. 159 

Alps, Lunar 72 

Altai, The 72 



TAGE 

Altair. . . .163, 170, 171, 172, 179, 180 

Altar, The 199 

Aluminium 61 

America 22, 144 

American Hemisphere, The 144 

Anaxagoras 11 

Anaximander 11 

Andromeda 146, 161, 165, 170 

Andromedae, y 146, 150 

Andromedes, The 144, 145, 140 

Angstrom 45 

Ann Arbor 102, 105 

Annual Parallax, The. .180, 181 . 225 

Annular Cavities 74 

Antares 171, 179 

Aphelion 214 

Apogee 213,216 

Apollo 169 

Apparent Motion 18 

Apparent Solar Day, The 27 

Apparent Time 27 

Appenines, Lunar 72 

Apple River 217 

Apsides 213, 214 

Aquarii, a ISO 

Aquarius 161, 165, 195 

Aquila 168, 172 

Arabia 16 

Arabians 16 

Arabic A- bronomy 16 

Arabic Terms 16 

Arabs, The 184 

Arago US 

Arcadia LSI 

Archer, P. Bcotl 

[he 161, UK 

Arctophylaz 



246 



IXDEX. 



PAGE 

Arcturas 138, 156, 159, 162, 163, 

171, 176, 178, 181, 191 

Argelander IT.'). 192 

Argo Navis 164 

Argus 102,199 

Ariadne 168 

Ariel 116 

Aries 162,164 

Aries, First Point of 218 

Arietis, e 150 

Aristarchus 13 

Aristyllus 13, 15 

Arrow, The 164 

Aselli, The 159 

Ash-gray Region, The 69 

Asteroid 104 

Asteroids 21, 22 

Astrology 10 

Astronomical Day 27 

Athens 12 

Atlantic Ocean 69 

August Meteoroids, The 144 

Auriga ....168, 170, 171 

Aumra Borealis, The 52 

Axes, Lunar 77 

Azimuth Circles 16 



Bacchus , 

Bailie 

Balance, The .. . 
Ball, Professor. 

Balsam 

Barium 

Bayer 



163, 

.64, 181, 242, 



Bee-hive, The 159, 

Bentley 

Berlin 

Berlin Year-book 

Bessel 

Beta Centauri 

Beta Crucis 

Betelgeuse 159, 162, 

Biela's Comet,l30, 133, 145, 146, 
Bilk 



168 
98 
165 

243 

84 
61 
156 

171 

8 

117 

llC) 

181 
179 
179 

170 
L50 
105 



PACK 

Birmingham 189 

Birth of Christ, The 33 

Bissextile 33 

Bithynia 13 

Bithynian 15 

Blue 40 

Bode 103,183 

Boiling Springs 97 

Bolides 146 

Bond, Professor ..112, 138, 204, 205 

Bond, The elder 22 

Bond. The younger 22 

Bonn 175 

Book of Job 156 

Bootes 156, 159, 168, 171 

Bootes, /j 150 

Boron 53 

Boston 217 

Bradley 21 

Bredichin 134, 135, 136 

Brewster 44 

Bromides of Silver 203 

Brorsen's Comet 130 

Buffham 115 

Burnham, S. W 22,183 

Bunsen 40, 45 

Cadmium 61, 203 

Cadmium Iodide 203 

Caesar, Julius 29, 32 

Caesium 45 

Calcium 61 

Calendar Moon 36 

Calendar, The 25, 32-39, 81 

Calendar Year 35 

Calippic Cycle 12 

Calippic Period 31 

Calippus 12,31 

Calippus Mountain 73 

Callisto 109, 167, 168 

Cambridge 204 

Camelopardalus 164 

Camelopard, The 164 

Camera, The 204 



Index. 



247 



PAGE 

Camera Obscura, The 204 

Cancer 159, 165 

Canes Venatici 163 

Canis Major 162, 167, 170, 171 

Canis Majoris, a 156 

Canis Minor 162, 167, 170, 171 

Canopic 12 

Canopus 178, 185, 201 

Cape Clouds, The 202 

Capella..l63, 170, 171, 179, 181, 191 

Capricornus 161, 165 

Carbon 53 

Carbonic Acid 122 

Carpathian Mountains, The ... 72 

Carrington 49, 51 

Casatus Mountain - 73 

Cassegrainian Telescope, The. 86 

Cassini, Dominicus 21, 153 

Cassini, Jacques 112, 113 

Cassiopeia 158, 165, 170, 171 

Castile 17 

Castor 160, 167, 170, 171 

Castor-oil 84 

Catasterisms 156 

Caucasian Mountains, The. . . . 72 

Cavendish 98 

Caverns, Lunar 74 

Celestial Chemistry 40 

Celestial Measurements 218-230 

Celestial Index 81 

Celestial Laws 207-217 

Celestial Photography 23, 46, 

203-206 

Celestial Physics 22, 40 

Celestial Pole 213, 219, 229 

Centaur, The 164, 195 

Centaurus 164 

Central Mountains 72 

Centrifugal Force 77, 97 

Centrifugal Limit 234, 235 

Cepheus. 164, 165 

Ceres 104 

Cerium 61 

Cetus , 161, 189 



PAGE 

Chaldea 7 

Chaldeans, The 7 

Charles' Wain 157 

Charioteer, The 163 

Chemical Plate 23 

Chemical Rays 65, 203 

Childrey 153 

China 9 

Chinese, The 7, 9 

Chladni 142 

Chloride of Silver 203 

Chromium 61 

Chromosphere, The 47, 53, 60 

Chromatic Aberration 83, 84 

Circle, The 209 

Cirri 50 

Civil Time 27 

Civil Tear, The 29, 30, 32 

Clairaut 130 

Clark, Alvan 86 

Clazomense 11 

Clepsydra, The 8 

Climate 88 

Clinton 105 

Cloud Theory, The 50 

Cloven flat Disc, The 200 

Cnidus, The.- 12 

Coal-sack, The 199 

Cobalt 61, 148 

Coggia 136 

Colchis 167 

Collimator, The 42 

Collodion 139, 203 

Collodion Film, The 203, 204 

Color 22 

Colored Stars 188-190 

Columba 162 

Coma Berenices... 162, 163, 167, 170 
171, 194, 199 

Comets 22, 129-140 

Common, Ainslie 205 

Common Salt 42, 45 

Conon 168 

Conservation of Areas, The . . . 236 



248 



Index. 



PAGE 

Conservation of Energy, The . . 242 
Conservation of Moment of 

Momentum, The 241 

Constellations, The 156-173 

Convection 47 

Convection-currents 53 

Convolvuli 58 

Copernican System, The 16, 20 

Copernican Theory, The 13, 19 

Copernicus 17, 18, 20 

Copernicus Mountain 74 

Copper 61 

Cordilleras, The 72 

Corona Australia 164 

Corona Borealia...l59, 182, 163, 168 
Corona, The.. .22, 47, 58, 60, 61, 153 

Cornu 98 

Corvua 163 

Counter Glow, The 152, 15;} 

Council ..f Nice 36, 37 

Crab, The 159, 165 

Cracow 17 

Crane, The 164 

Crape Ring, The 1 11, 112 

Crater, The 163 

Craters, Lunar 74 

Crete 168 

(row. The 163 

Crown Glass 84 

Crystalline Spheres 12 

Cup, Tin- 163 

Curtius Mountains 73 

Cyanide of Potassium 204 

Cycladee, The 166 

Cyclonic Theory, The 51 

Cygnus 163 

Cytherean Seasons 126 

Cyzicus 12 

Daguerre 203 

Daily Parallax 221 

D'Alembert Mountains 72 

Danae 166 

Danube, The 98 



PAGE 

D'Arrest's Comet 130 

Davy 203 

Dawes 133 

Daj^, The 25 

Declination, The 218 

Deferent, The 14 

Deimos 101 

Delambre 15, 21, 63 

Delphinus 163, 170 

Delta Equulei 183 

Demon, The 184 

Deneb 192 

Denebola 158, 163 

Denderah 10 

Denderah, Zodiacs of 10 

Denmark 18 

Denning, W. F 116, 146 

Derham 50 

Dessau 51 

Diana 167 

Diathermanous 47 

Diffraction 61 

Digit 58 

Digits 58 

Dione 113,114 

Dipper, The 157, 159, 170, 171 

Dispersion 83 

Diurnal Parallax 221 

Division of Time, The 25-32 

Doberck 183 

Dog-Star, The 29, 30, 162, 167 

Dolland 84 

Dolphin, The 163 

Dominical Letter, The 38, 39 

Donati 133, 136, 138, 139 

Doradus 164 

Dorf el Mountains 72, 73 

Dove, The 162 

Draco 162 

Draeonis, a 162 

Dragon, The , .162, 197 

Draper 23, 108, 204, 205 

Driessen 104 

Dublin 86 



IXDEX. 



249 



PAGE 

Dunkin 192 

Dunsink 181 

Dry Gelatino-bromide Plates.. 139 

Durham 200 

Duodecimal Division 8 

Eagle, The,152, 163, 170, 171, 172, 199 

Earthshine 68 

Earthquakes 97 

Earth, The 97-99, 120 

Easter 33, 36 

Easter Day 36, 81 

Easter Sunday 36 

Ebn-Junis 16 

Eccentric, The 14 

Eccentricity, The 210, 214 

Eccentrics 14 

Eclipses 54 

Eclipses, Lunar 78-79 

Eclipses, Solar 54-59 

Ecliptic, The 29, 54, 212 

Edinburgh 119 

Egyptians, The 7, 10, 11, 29, 30 

Electric Arc 45, 46 

Electricity 24, 122 

Electric Marking-apparatus ... 24 

Electrical Repulsion 61, 135 

Elkin, Dr 182 

Ellipticity 211 

Ellipse, The 209 

Ellipsoid of Revolution 97 

Enceladus 113, 114 

Encke 63, 131, 132 

Encke's Comet. . . .130, 131, 132, 133 

Epact 35, 36 

Epicycle 14 

Epicycles 14 

Equation of Time 27 

Equatorial, The 85, 199 

Equinoctial Spring-tides 216 

Equinox 15 

Equinox, Autumnal 80 

Eratosthenes 13 

Ericsson 80 



PAGE 

Eridanus 164 

Esneh 10 

Esneh, Zodiacs of 10 

Eta Argus 178, 185, 241 

Ether 203 

Ethiopia 165 

Eudoxus 12 

Euler 83 

Europa 109 

Europe 16, 144 

Euryale 166 

Eurydice 169 

Evection, The 9, 15, 213 

Evening Star, The 94 

Evergetes 167, 168 

Eye-lens 82, 85 

Eye-piece 82, 83, 84, 85, 86, 87 

Eye-piece, The Negative 85 

Eye-piece, The Positive 85 

Fabricius 50, 184 

Faculse 49 

Faye 51, 64, 244 

Faye's Comet 130 

Field-lens 85 

Fireballs 146, 147 

Fishes, The 161, 165 

Fissures 49 

Fizeau. 63 

Flamsteed 62, 157 

Flint Glass 42, 84 

Flood Tide 214 

Flora 103 

Fluorescent Bodies 44 

Fly, The 164 

Flying Fish, The 164 

Focus 82, 86, 87 

Fomalhaut 16, 161, 170, 179 

Forbes, Professor 119 

Foucault, Leon 45, 63 

Fox, The 164 

Frauenburg 17 

Frauenhofer 23, 44, 45 

Frederick II 17 



250 



Index. 



PAGE 

French Opticians 83 

Friday 32 

Foody Bay 317 

Galactic Circle 190, 200 

Galaxy, The 162, 199-201 

Galileo 20, 50, 82, 207, 208 

Galle, Dr 117, 146 

Ganymede 109 

Gassendi 94 

GaoBfl 104,136 

Qegeoscheio, The 152, 158, 154 

Gelatine 139 

Gemini 100, 165, 171 

Geminids, The 145 

Gemioorom, , 150 

GeoCeotrlC Parallax, The 221 

Geologists 97 

< i.riiianv 17 

GUI, Dr 84, 181 

Giraffe, The 164 

Glaseoap 68 

Glasgow 108 

Glass Prism 40 

Gnomon, The 8 

Goat, The 161, 165 

Golden Number, 1 he 34, 35, 36 

Gore, J. K 183 

Gordons, The 165 

Gravity 61 

Gravity, The Laws of 2(<7 

Gravity Hypothesis, The 211 

Great Bear, The 156, 158, 167 

Great Dog, The 162, 167, 177 

Great Pyramid, The 10 

Grecians, The 11 

Greece 11 

Greek Alphabet 155 

Greek Astronomy 11 

Green 40 

G reeohoose 47 

Greenwich 230 

Gregorian Calendar, The 36 

Gregorian Rule, The 33, 37, 39 



PAGE 

Gregorian Telescope 86 

G regory, James 86 

Gregory Xlii 33 

Grindstone Theory 200 

Grutb, Howard 85 

Groombridge 192 

Guillemin 112, 181, 191 

Habitablenesa 121-128 

Habitability 121-128 

Hades 169 

Bakem 16 

Hakemite Tables, The 16 

Hall, Asaph 22,101 

Hall, Maxwell 118 

Hall, Professor 86, 181 

Halley, Edmund 21,129,130 

Bailey's Comet 130, 131, 135 

Hammerfest 98 

Hansen 63,70 

Harding 104 

Bare, The 162 

Harkness 64 

Harp, The 162, 169 

Harrington, Professor 102, 103 

Harvest Moon, The 80 

Hastings, Professor 61 

Heating Pvays, The 203 

Hebrus, The 169 

Heliocentric Parallax, The 225 

Heliocentric System, The . 12 

Heliocentric Theory, The 12 

Heliometer, The 89 

Helmholz 65 

Hencke 104 

Hen-coop, The 160, 179 

Henderson, Thomas 181, 182 

Henry Brothers, The 115 

Hercules 162, 192, 194 

Herodotus 11 

Herschelian Principle, The 87 

Herschelian Telescope, The. .. 87 
Herschel, Prof. Alexander S. . . 145 
Herschel, Sir John, 45, 50, 51, 74, 202 



Index. 



251 



PAGE 

Herschel, William. . .21, 51, 86, 113, 
114, 116, 123, 198, 199, 200 

Hi 9 

Hindoos, The 7, 8 

Hipparchus 13, 14, 15, 16, 34, 94 

Ho '... 9 

Hoedi, The 163 

Holden, Professor 22, 181 

Homer 156 

Horizontal Parallax, The. . ..62, 221 

Horn Silver 203 

Huggins, Dr 23, 108, 134, 140, 

187, 188, 197, 205 
Humboldt. .13, 51, 152, 153, 156, 186 

Hunting Dogs, The 163 

Hurricanes 64 

Huyghens 21, 113, 207 

Huygheus, Mountain 73 

Hyades, The 160, 194 

Hydro-carbon Spectrum, The . . 134 

Hydrogen 23, 60, 61, 136, 178 

Hydra 162 

Hyperbola, The 136 

Hyperion.... 113, 114 

Incidence, Angle of 75 

India 8,98 

Indiana University 105 

Indian Tables 8 

Indian, The 164 

Indiction Cycle 38 

Indiction, The 38 

Indigo 40 

Inferior Planets, The 91 

Inner Planets, The 91 

Insulated Mountains 72 

Intercalary Addition Si 

Intercalation. 31 

Io 109 

Iodide of Silver, The 203 

Ionia 11 

Irak 202 

Ireland 87 

Iron ,43,61,136 



PAGE 

Iron-falls 146 

Iron Lines , 44 

Ismailia 98 

Isochronism 69, 70 

Janssen 23, 53, 140, 205 

Japetus 113, 114 

Java 122 

Jews, The 31 

Job's Coffin 163,170 

Jovian System 21 

Julian CaleDdar 32 

Julian Rule 33 

Julian Period 38 

Julian Year 29, 33 

Juno 103, 104, 165 

Jupiter. . . .9, 20, 21, 32, 90,106-8, 120 
Jupiter's Satellites 109 

Kant 200 

Kentucky 58 

Kepler, John. . .19, 20, 103, 208, 209 

Kepler's Laws 208-209, 223 

Kids, The 163 

Kirchhoff 23, 40, 43, 45 

Kirkwood, Daniel 22, 105, 243 

Knudsthorp 18 

Konigsberg 181 

Krakatao 149 

Labyrinth, The 168 

Lacerta 164 

Lagrange 21, 212 

Lahire 62 

Lalande 21, 50, 130 

Lambert 69 

Lamont, Dr. John 51 

Langley, Professor 23, 50, 58 

Laplace. .8, 21, 114, 136, 153, 198, 212 
Laplace, Hypothesis of.. 65, 233-244 

Lassell, W 113, 116, 118 

Latitude, The 218, 229 

Lead 61 

Leap-year , 33 



252 



IXDEX. 



PAGE 

Leibnitz 21 

Leibnitz .Mountain 72, 73 

Leo 1-13, 158, 165, 170 

Leo Minor 168 

Leonids, The 142, 145. 146 

Leonis, /3 150, 158 

Leonis, y 150 

Lepua 162 

Letronne 7 

Leverrier ...21, 22, 63, L03, 117, 144 

Lewis, Professor 154 

Lexell 114 

Libra 163, 165 

Libration, Diurnal 77 

LibratioD in Latitude 77 

LibratioD in Longitude 77 

Librationa 77 

Lick Observatory, The... 

Life, The Conditions of 121 

Light 109, 228 

Lilientlial 101 

Lincobishire 21 

Line, The D 42 

Linnaeus 128 

Lion, The 143, 158, 165 

Lithia 45 

Little Bear, The 158 

Little Dog-Star, The 162 

Little Lion, The 163 

Livy 146 

Lizard, The 164 

Lockyer 23, 60 

Longitude, The 318, 230 

Lord Rosse 79, 87 

Luminous Bands 76 

Luna Cornea 203 

Lunar Cycle 34, 35 

Lunar Distances 230 

Lunar Eclipses 230 

Lunar Month 30, 35, 66 

Lunar Period 22 

Lunar Perturbations 213 

Lunar Theory 22 

Lunar Tides 217 



PAGE 

Lunar Year 35 

Lunation 30, 54, 66 

Luui-solar Cycle 9, 12 

Lupus 164 

Luther 105 

Lynx, The 164 

Lyra 162,169,170 

Lyne, a 150,156,171 

Lyrids, The 144 

Madler 192 

Magellan 202 

Magellanic Clouds 198, 201-202 

Mairne<ium 61 

Magnifying Powers, The 82 

Magstadt 19 

Major Planets, The 91 

Mammoth Cave, The 58 

Manganese 61 

Manger, The 159, 170, 194 

March Equinox, The 218 

Mare Crisium 71 

Marius, Simon 50 

Mars 9, 19, 21, 22, 32, 62, 63, 90, 

99-102, 120, 127, 128, 223 

Marsh Gas 134 

Martian Continent 101 

Martian Day 101 

Martian " Seas " 101 

Mart ian Surface 101 

Martian World 128 

Martian Year 128 

Mathematics 21 

Maxwell, James Clerk 112 

Mean Solar Day 27 

Mean Solar Time. , . . . 27 

Mean Time 27 

Mechanics 21 

Mechanism of the World ..231-244 

Medusa's Head 165, 166 

Medusa, The 166 

Mercury . . .9, 20, 32, 90, 92-94, 120, 

124, 133 

Meridian Circle, The 89 



IXDEX. 



253 



PAGE 

Meridian, The 230 

Mersenne 86 

Meteorites 146, 148, 149 

Meteoroids 142, 144, 154 

Meteorolites 146 

Meteors 22,141 

Meton 12,31,34 

Metonic Cycle 9, 12, 31, 34, 38 

Mexico 152 

Micrometer .72, 88, 89 

Milan Observatory 101 

Miletus 11 

Milky Way, The . .152, 195, 199-201 

Mimas 113, 114 

Mineralogists, The 149 

Minerva 166 

Minerva's Temple 166 

Minoceros 162 

Minor Planets, The 91, 102-105, 

124, 223, 224 

Minotaur, The 168 

Mira 161,184 

Moment of Momentum, The. 241-24 4 

Monday. ..'. 32 

Montuela 9 

Moon, The 32, 66-81, 120, 223 

Moscow 135 

Motion of Translation 77, 137 

Motion, The Laws of 207 

Mount Alban 146 

Mount Hamilton 86, 183 

Mu 143 

Musica 164 

Nadir 16 

Naples 122 

Napoleon 10 

Nasmyth, James 47 

Nautical Almanac 57, 230 

Neap Tides 216 

Nebulas, The 194, 195-199 

Nebular Hypothesis, The 65, 

233-244 
Neptune . . .21, 90, 117-119, 120, 165 



PAGE 

Nereides, The 165 

New Astronomy, The 22, 23, 40 

New Caledonia 217 

Newcomb, Simon.22, 63, 64, 181, 244 

New South Wales 139 

Newton £1, 86, 207, 211 

Newton, Crater of 73 

Newton, Professor 143 

Newtonian Telescope 87 

New York 204, 21? 

Nicasa 13 

Nice 115, 116 

Nicholas of Cusa 17 

Nickel 61, 148 

Niepce 203 

Night-shades 58 

Nile, The ..12,167 

Nitrogen 23, 122 

Nocta 164 

Nodal Points 213 

Node, The Ascending 54 

Node, The Descending 54 

Nodes, Moon's 54, 213 

North America 142 

Northern Cross, The 163 

Northern Crown, The 159, 171 , 

178, 195 

November Meteoroids, The 144 

November Meteors, The 143 

Nubecula Major, The.. 198, 201, 202 

Nubecula Minor, The 201, 202 

Nucleus, The 47, 49, 129 

Numa 32 

Nutation 212, 213 

Oberon 116 

Objective, The 82, 83 

Oblate Spheroid 97 

Observatory 24 

Occultation 76, 230 

Ocean of Tempests, The 71, 72 

Olbers 104, 105, 143 

Old Astronomy, The 22, 40 

Olefiant Gas 134 



254 



Index. 



PAGE 

Olmstead 143 

Oniicn .n Ceti 183 

Ophiucua 163,199 

range : 40 

Orbital Moment of Momentum, 

The 242 

Orion.. ..156, 159, 166, 167, 170, 171, 
179, 189, 199, 205 

(Moulds, The 145 

Ononis, v ISO 

Orpheus 168 

( Hir.tl Mountains, The 72 

Outer Planets, The 91 

Owl. The 164 

Oxygen 122 

Palermo 104 

Palisa 104 

Pallas lu:i, 104, 105 

Parallax 219-238 

Parallax, Absolute 225 

Parallax, Differential 225 

Parallax, Diurnal 221 

Parallaxes 222 

Parsoustown 87 

Peacock, The 164 

Pegasus 160, 161 

Pegasus, Great Square of. .160, 161, 

170, 171 

Peine. Professor 22, 137 

Penumbra 49 

Perigee, The 213,216 

Perihelion 132, 214 

Periodic Perturbations 211 

Perrotin 115 

Pereei, (J 150,184 

Persei, e 150 

Pereei, >/ 150 

Perseids, The 144, 145, 146 

Perseus, Constellation of.. 14+, 161, 
165, 170, 195 

Persia 8 

Peters 104, 144, 1S1 

Phases 66,67 



PAGE 

Phobos 101 

Phoenix, The.... 164 

Phosphorus 95, 148 

Photographer's Plate. 140, 206 

Photography 23, 44, 63, 108 

Photometer, The 175 

Photosphere, The 47, 50, 52, 53 

Photospherie Clouds 50, 53 

Piazzi 21, 104 

Picard, Jean 21, 62 

Pickering, Professor. . . .23, 102, 176 

Pisa 20 

Pisces 161, 165, 189 

Piscis Australis 161, 170 

Planetary Perturbations 211 

Planetary Theory 15 

Planetoids, The 102-105 

Planet s, The 90-120 

Phmo -concave Lens 84 

Plato 74 

Pleiades, The.. 156, 160, 162, 170, 194 

Pliny 11 

Plough, The 157 

Plutarch 168 

Pluto 166, 169 

Pointers, The 157, 158 

Polar Distance, The 218, 219 

Polaris 157, 158, 179, 181, 191 

Polariscope, The 75 

Polar Star, The 229 

Pole, The 157, 170, 201, 229 

Polish Bull, The 164, 171 

Pollux. . .160, 167, 170, 171, 179, 180 

Poppies 58 

Pores 50 

Portuguese, The 202 

Potassium 61 

Pozzuolo Grotto, The 122 

Praesepe, The 159, 194 

Precession 9, 212 

Precession of the Equinoxes, 

The 15,29,212 

Priestley 203 

Primary Colors 41 



Index. 



255 



PAGE 

Principia 207 

Pritchett, Professor 108 

Procyon 162, 170, 171, 179 

Proserpine 169 

Proto-sulphate of Iron 204 

Prussia 17 

Ptolemaic System 17 

Ptolemy 7, 8, 9, 15, 16, 17, 94 

Pulkowa Observatory 86 

Purbach, George 17 

Pyramids 10, 11 

Pyrogallic Acid 204 

Pythagoras 11, 13 

Quadratures, Tides of the 216 

Quito 152 

Radii Vectores 210 

Radius Vector 210 

Rainbow, The 40 

Ram, The 162, 164 

Red 40,41,83 

Reflector, The 82, 86 

Reflectors. 82 

Refracting Prism, The 22 

Refracting Telescope, The 82 

Refraction 40, 41, 83, 226-227 

Refractor, The 82, 83 

Refractors 82, 85 

Refrangible 41 

Regiomontanus 17 

Regulus 158, 170, 171, 179, 180 

Relative Parallax 225 

Resisting Medium 131, 132 

Respighi 23 

Reticule, The 164 

Retrograde Motion 118 

"Revolutions of the Heavenly 

Bodies" The 18 

Revolving Cylinder, The 24 

Rhea 113, 114 

Rice Grains 47, 50 

Richer 62 

Rigel 16, 159, 162, 179 



PAGE 

Right Ascension, The 218 

Right Ascensions 89 

Rills 76 

Rio Janeiro 140 

River Po, The 164 

Roberval 112 

Rome 9, 10, 17, 146 

Rosse, Lord 79, 87 

Rotational Moment of Momen- 
tum, The 212 

Rubidium 45 

Rutherfurd 205 

Sabine 51 

Sagitta 164 

Sagittarius 161, 165 

Saint-L6 21 

Salts of Silver, The 203 

Samos 11, 13 

Saros 7, 11 

Saturn. 9, 20, 32, 90, 110-114, 120, 181 

Saturn 's Dusky Ring 22 

Scaliger, Joseph.- 38 

Schafarik 115 

Schaubach 8 

Scheele 203 

Schemer 21 

Schiaparelli. .101, 115, 127, 136, 143, 
144 

Schmidt, Dr 186 

Schomberg, Cardinal 18 

Schwabe, Heinrich 51, 52 

Scintillation 173, 188 

Scorpio 163, 165, 171 

Scorpion, The 163, 165 

Scutum Sobieski 164 

Sea of Clouds 71, 72 

Sea of Crisis 71 

Sea of Humors 71 

Sea of Nectar 71, 72 

Sea of Plenty 71, 72 

Sea of Rains 71, 72 

Sea of Serenity 71, 72 

Sea of Tranquillity 71 



256 



Index. 



PAGE 

Sea of Vapors 72 

"Seas" 71 

Becchl 23, 134, 187 

Secchi'e Hypothesis 65 

Secular Perturbations 212 

Selective Absorption 43 

Serpent, The 163 

Sextans 1G4 

Sextant, The 164 

Ship Argo 164 

Shooting-stars 141-150 

Sickle, The 158, 159, 162, 163, 

170, 171 

Sidereal Day 25 

Sidereal Revolution 30, 70 

Sidereal Year 28 

Siderites 147 

Silicon 53 

Silver 203 

Sins Hi7 

Blrius 80,92, 156, tea, 167, 170, 

171, 175, L77, 181, l'.'l, 103 
Slag Theory, The 50 

Smith, l'iazzi 10 

Sobieski'e Shield 164 

Sodium 23, 41,43,44,45,61 

Sodium Line, The 42, 43 

onBtitution, The 61 

Solar Cycle, The 36, Si 

Solar Day. The 25, 27, 84 

Solar Eclipse, Annular 57 

Solar Eclipse, Partial 57 

Bolar Eclipse, Total 57 

Solar Eclipses 54-59 

Solar Physics 22,40 

Solar Spectrum, The. .42, 43, 41, 4.'. 

Solstices, The 8, 18 

Sosigenes 32 

Sothic Period 29 

Sothis 29 

Source of Solar Heat 65 

South America 69 

Southern Cross, The 164, 199 

Southern Fish, The 161 



PAGE 

Southern Triangle, The 164 

Spectroscope, The. . . .22, 23, 40-45, 

46, 133, 192 

Spectrum Analysis. . . .22, 40, 43, 45 

Spectrum, The 41, 44 

Spectrum, The Solar 61 

Speculum 83, 86, 87 

Spica 163, 171, 179, 180 

Star Clusters 193-195 

Stare, The 173-193 

Stewart, Balfour 45 

Stheno 166 

Stellar Masses 192-193 

Stellar Motion 190-193 

Stellar Parallax 181, 225 

Stellar Photometry 23,175-177 

Stellar Spectra 187-188 

Stokes, Professor 45 

Stone 63, 64 

Stone-falls 146 

Strontium 41, 61 

Struve, Otto 116, 183, 192 

Stylus, The 29 

Sun, The 32,46-65 

Sunday 32 

Sunday Letter 38 

Sun's Distance 61, 223 

Sun's Heat 64 

Sun's Temperature 61 

Surya Siddhanta 8 

Swan, The. . . .152, 171, 172, 181, 199 

Syrian Coast, The 165 

Syzygies, Line of the 213 

Syzygies, Tides of the 216 

Talbot 45,203 

Taurus, Constellation of... 104, 160, 
164, 170, 171, 189 

Taurus Poniatowski 164, 172 

Tehbutt, John 139 

Tebbutt's Comet 139, 204 

Telescope, The 20, 82-89 

Tempel's Comet 131 , 134, 144, 

145, 146 



Index. 



257 



Temperature of Space 148 

Temporary Stars 185-186 

Terminator, The 74 

Tethys 113,114 

Thales 11 

Thermo-electric Pile 79 

Thermometer, The 79 

Theseus 168 

Thollon 115 

Thorn 17 

Thursday 32 

Tidal Action 70 

Tides, The 81, 98, 214-217 

Timocharis 13, 15 

Titan 113,114 

Titania 116 

Titanium 61 

Tithonic Kays 295 

Todd, Dr 63, 64, 119 

Totality 58,78 

Toucan 164, 195 

Transit Instrument 27, 85 

Transit of Venus, The 62, 96, 

223, 224 

Triangles, The 164 

Triangula? 164 

Tropical Year, The 29, 33, 34 

Tropics, The. 29, 152 

True Time 27 

Tuesday 32 

Tuttle's Comet 131 

Twilight 151, 227-228 

Twins, The. . .114, 160, 165, J 67, 170 
Tycho Brahe 18, 19, 186 

Umbra 49 

Umbriel 116 

Unicorn, The 162 

Universal Gravitation 21, 211 

Uranite 124 

Uranium 61 

Uranus 21, 90, 114-117, 120 

Ursa Major 157, 167, 170 

Ursa Minor 158 



PAGE 

Variable Stars 183-185 

Vega. . . .156, 162, 163, 170, 171, 172, 
176, 177, 179, 181 

Venus 12, 20, 21, 32, 62, 63, 90, 

94-96, 120, 125, 223 

Vernal Equinox, The 36, 218 

Vesta 102, 103, 104 

Vienna 86, 104, 144 

Violet 40,41,83 

Virgin, The 163, 165 

Virgo 163, 165, 171 

Vistula 17 

Vogel 108, 134 

Volcanoes 74, 97 

Von Asten 93, 133 

Vulcan 94 

Vulpecula 164 

Washington 119, 181, 230 

Washington Equatorial, The. . . 85 
Washington Refractor, The ... 86 

Water-bearer, The 161, 165 

Water-snake, The 162 

Watson 105 

Weather, The 51, 52 

Wedgwood 203 

Wednesday 32 

Week, The 31 

Weiss 144, 146 

Whale, The 161, 199 

Wheatstone 63 

Whirlpool Theory 51 

White Ox, The 202 

Willow-leaves 47 

Wilson 50 

Winnecke's Comet 131 

Wolf, Rudolph 51, 52 

Wolf, The 164 

Wollaston 44 

Wonderful, The 184 

Woolsthorpe 21 

Wright, Professor .112, 154 

Wright, Thomas 200 

Wurtemberg 19, 208 



L\~>S 



Index. 



PAGE 

Yale College 143 

lard and Ell, The 160 

Year, The..- 25 

Yellow 40 

Yorkshire 146 

STonng 23,60, 84, 115, 191 

Zenith, The 16, 50, 70 

Zenith Distances, The 89 

Zet-u Aquarii 183 



PAGE 

Zinc 61 

Zodiac, The 7, 18, 151, 171 

Zodiacal Band, The 152, 153 

Zodiacal Cone, The 152 

Zodiacal Highway, The . . . .171, 173 

Zodiacal Light, The 151-155 

Zodiacal Road, The 171, 172 

Zodiacs 10 

Zollner 23, 134 



